Fixed the memory usage explanation of categorical in gotchas from O(n…
…m) to O(n+m)

- [ ] closes #17705 


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=h1) Report
> Merging [#17736](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/408ecd21759979f94952a48a0e4a2b2a608ee84d?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17736/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17736      +/-   ##
==========================================
+ Coverage   91.23%   91.23%   +<.01%     
==========================================
  Files         163      163              
  Lines       49796    49796              
==========================================
+ Hits        45430    45433       +3     
+ Misses       4366     4363       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.33% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=footer). Last update [408ecd2...439cefc](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=h1) Report
> Merging [#17736](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/408ecd21759979f94952a48a0e4a2b2a608ee84d?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17736/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17736      +/-   ##
==========================================
+ Coverage   91.23%   91.23%   +<.01%     
==========================================
  Files         163      163              
  Lines       49796    49796              
==========================================
+ Hits        45430    45433       +3     
+ Misses       4366     4363       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.33% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=footer). Last update [408ecd2...439cefc](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=h1) Report
> Merging [#17736](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/408ecd21759979f94952a48a0e4a2b2a608ee84d?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17736/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17736      +/-   ##
==========================================
+ Coverage   91.23%   91.23%   +<.01%     
==========================================
  Files         163      163              
  Lines       49796    49796              
==========================================
+ Hits        45430    45433       +3     
+ Misses       4366     4363       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.33% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=footer). Last update [408ecd2...439cefc](https://codecov.io/gh/pandas-dev/pandas/pull/17736?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!