Remove experimental warning from custom offsets
Per discussion here: https://github.com/pandas-dev/pandas/pull/17554#pullrequestreview-63229522

- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I edited the docstring descriptions slightly.  Feel free to suggest cleaner descriptions, if my edits don't suffice.  Should there be a whatsnew entry related to removing the experimental status?


AFAIK, you do not need to add a `whatsnew`. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=h1) Report
> Merging [#17584](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6630c4eddf2762d519507304ad73de189a7e0c6c?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17584/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17584      +/-   ##
==========================================
- Coverage   91.22%    91.2%   -0.02%     
==========================================
  Files         163      163              
  Lines       49625    49625              
==========================================
- Hits        45270    45261       -9     
- Misses       4355     4364       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <ø> (ø)` | :arrow_up: |
| #single | `40.19% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17584?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/17584?src=pr&el=footer). Last update [6630c4e...ba1c154](https://codecov.io/gh/pandas-dev/pandas/pull/17584?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/17584?src=pr&el=h1) Report
> Merging [#17584](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6630c4eddf2762d519507304ad73de189a7e0c6c?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17584/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17584      +/-   ##
==========================================
- Coverage   91.22%    91.2%   -0.02%     
==========================================
  Files         163      163              
  Lines       49625    49625              
==========================================
- Hits        45270    45261       -9     
- Misses       4355     4364       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <ø> (ø)` | :arrow_up: |
| #single | `40.19% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17584?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/17584?src=pr&el=footer). Last update [6630c4e...ba1c154](https://codecov.io/gh/pandas-dev/pandas/pull/17584?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!