DOC: use single back-ticks for variable, module, function, and class names
EDIT I've rewritten this issue after Joris' comment.

----

Pandas' docstrings are somewhat inconsistent in using single or double back-ticks, e.g. [isin](https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.isin.html?highlight=isin#pandas.DataFrame.isin) has both
```
`values`
```
and
```
``values``
```

In keeping with the [numpydoc docstring guide](https://numpydoc.readthedocs.io/en/latest/format.html), which reads

> Use *italics*, **bold** and ``monospace`` if needed in any explanations (but not for variable names and doctest code or multi-line code). Variable, module, function, and class names should be written between single back-ticks (`numpy`).

, it might be good update docstrings which don't follow this convention, as well as to mention this convention in the pandas docstring guide.