DOC: Document business frequency aliases
Follow-up to #16978.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17028?src=pr&el=h1) Report
> Merging [#17028](https://codecov.io/gh/pandas-dev/pandas/pull/17028?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/dd1852d59e51e28c2f9b589eefa1916ef5d9bdc9?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/17028/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17028?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17028      +/-   ##
==========================================
+ Coverage      91%      91%   +<.01%     
==========================================
  Files         161      161              
  Lines       49304    49304              
==========================================
+ Hits        44867    44870       +3     
+ Misses       4437     4434       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.19% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17028?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17028?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17028?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.76% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17028?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.05% <0%> (+1.81%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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