DOC/CLN: Fix errors in DataFrame docstrings
A few small edits to `DataFrame` docstrings
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24952?src=pr&el=h1) Report
> Merging [#24952](https://codecov.io/gh/pandas-dev/pandas/pull/24952?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/37a224d216e8d49a05681e2c87482bd8e67385fc?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24952/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24952?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24952   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52398    52398           
=======================================
  Hits        48407    48407           
  Misses       3991     3991
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `42.88% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24952?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/24952/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.92% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24952?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/24952?src=pr&el=footer). Last update [37a224d...2072a58](https://codecov.io/gh/pandas-dev/pandas/pull/24952?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/24952?src=pr&el=h1) Report
> Merging [#24952](https://codecov.io/gh/pandas-dev/pandas/pull/24952?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4d44a2aff08c3075d9472e361bd7448a0472494e?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24952/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24952?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24952   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files         173      173           
  Lines       52831    52831           
=======================================
  Hits        48457    48457           
  Misses       4374     4374
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.28% <ø> (ø)` | :arrow_up: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24952?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/24952/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.87% <ø> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/24952/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `94.16% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24952?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/24952?src=pr&el=footer). Last update [4d44a2a...3ecebbd](https://codecov.io/gh/pandas-dev/pandas/pull/24952?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Can you merge master? Thanks @dsaxton !