Lint rule to catch incorrect sphinx directives
#### Code Sample, a copy-pastable example if possible
None. Documentation / build issue.

#### Problem description

There are few places where tag "versionadded" misses one semicolon, making it a sphinx comment instead of directive:
```
pandas/core/frame.py
4516:            .. versionadded: 0.18.0
4679:            .. versionadded: 0.16.1

pandas/core/generic.py
968:            .. versionadded: 0.21.0

pandas/core/series.py
1629:            .. versionadded: 0.19.0
2216:            .. versionadded: 0.18.0

pandas/core/tools/datetimes.py
117:        .. versionadded: 0.18.1
143:        .. versionadded: 0.16.1
181:        .. versionadded: 0.20.0
187:        .. versionadded: 0.22.0

pandas/tseries/offsets.py
778:    .. versionadded: 0.16.1
882:    .. versionadded: 0.18.1
```

Correct way to use it: `.. versionadded:: x.xx.x"` (note two semicolons).

Apart from fixing it here and now, it would be useful to have a lint rule that warns contributors about this and similar issues in future (in docstrings and in docs).

#### Expected Output

Lint rule to catch incorrect sphinx directives.

#### Output of ``pd.show_versions()``
Doesn't matter.