DOC: Add DateOffsets to api.rst
- [x] closes #22447

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23405?src=pr&el=h1) Report
> Merging [#23405](https://codecov.io/gh/pandas-dev/pandas/pull/23405?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a2e599499667b256bc5b8b13a75f0601eccfd432?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/23405/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23405?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23405   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files         161      161           
  Lines       51184    51184           
=======================================
  Hits        47185    47185           
  Misses       3999     3999
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `42.23% <ø> (ø)` | :arrow_up: |


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 thanks. I actually think there might be another issue about this if you can find it (to close it) @jorisvandenbossche I don't quite follow your comment. IIRC `offsets` can be pulled in from the top level although these offsets live in `pandas/tseries/offsets/`?