DOC: update DataFrame.dropna's axis argument docs
Make `DataFrame.dropna`'s axis argument type deprecated warning in documentation easier to understand.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21652?src=pr&el=h1) Report
> Merging [#21652](https://codecov.io/gh/pandas-dev/pandas/pull/21652?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b35cb1c127aae894c2a1ee5ab2f16987b91e9000?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/21652/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21652?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21652   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         154      154           
  Lines       49558    49558           
=======================================
  Hits        45545    45545           
  Misses       4013     4013
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.27% <ø> (ø)` | :arrow_up: |
| #single | `42.02% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21652?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/21652/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.19% <ø> (ø)` | :arrow_up: |

------

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