DOC: inconsistent docs-string signatures for shared functions
xref #15580

related to this is our inconsistent treatment of the actual args in shared doc strings, e.g.

so the following have exactly the same arguments to the doc-string (except for the first 1-2 which are the axes: index, or index & columns).

The doc-strings themselves are correct, but the signature are hidden. I think easiest is simply to actually list out the args where they are defined (e.g in ``core/series.py`` for example).

```
Signature: pd.Series.reindex(self, index=None, **kwargs)
Docstring:
Conform Series to new index with optional filling logic, placing
NA/NaN in locations having no value in the previous index. A new object
is produced unless the new index is equivalent to the current one and
copy=False
```

```
Signature: pd.SparseSeries.reindex(self, index=None, method=None, copy=True, limit=None, **kwargs)
Docstring:
Conform SparseSeries to new index with optional filling logic, placing
NA/NaN in locations having no value in the previous index. A new object
is produced unless the new index is equivalent to the current one and
copy=False
```

```
Signature: pd.DataFrame.reindex(self, index=None, columns=None, **kwargs)
Docstring:
Conform DataFrame to new index with optional filling logic, placing
NA/NaN in locations having no value in the previous index. A new object
is produced unless the new index is equivalent to the current one and
copy=False
```

There are a bunch of functions like this.