DOC: Whatsnew 1.2.0 cleanup
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Cleanup of spelling, grammar, and links.

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cc @jorisvandenbossche, @dsaxton, @phofl: 

Using ``:meth:`.DataFrameGroupBy.quantile` `` (note the `.`) displays the text `DataFrameGroupBy.quantile` and sphinx finds the link to the proper page. Similar remarks apply to `Rolling` and others. From the [sphinx docs](https://www.sphinx-doc.org/en/master/usage/restructuredtext/domains.html):

> Also, if the name is prefixed with a dot, and no exact match is found, the target is taken as a suffix and all object names with that suffix are searched. 

I don't believe this was known about (at least to me!) and is different from what was aligned on in #37145, so I wanted to get thoughts here on which is preferred. While I've added many cross-links using this, `DataFrameGroupBy.quantile` is the sole example where I have replaced a preexisting link.

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Other notable changes that are worth a mention to make sure they are correct:
 * Numpy is always referred to as NumPy in plain-text (no backticks)
 * When a method exists on only Series and DataFrame, replaced unlinked reference with a reference to each. For example, ``:meth:`read_csv` `` becomes ``:meth:`DataFrame.to_csv` and :meth:`Series.to_csv` ``
 * dtypes and argument values are always double-backticked
 * Moved the following note from Other to Missing

> Bug in :meth:`Series.nunique` with ``dropna=True`` was returning incorrect results when both ``NA`` and ``None`` missing values were present (:issue:`37566`)
> Using `` :meth:`.DataFrameGroupBy.quantile` `` (note the .) displays the text DataFrameGroupBy.quantile and sphinx finds the link to the proper page  ..
> I don't believe this was known about (at least to me!) and is different from what was aligned on in #37145, so I wanted to get thoughts here on which is preferred

Ah, that looks nice. Do you know, does sphinx search in the possible link targets, or does it search in the pandas namespace? (just wondering, because it could also find `pandas.core.groupby.generic.DataFrameGroupBy` which is the actual location of the object definition, but it is documented as `pandas.core.groupby.DataFrameGroupBy`) > Ah, that looks nice. Do you know, does sphinx search in the possible link targets, or does it search in the pandas namespace?

If I'm reading the docs correctly, it's the same way references are usually resolved but in reverse order. I would have to assume that is searching possible link targets, but am not certain on this:

> Normally, names in these roles are searched first without any further qualification, then with the current module name prepended, then with the current module and class name (if any) prepended. If you prefix the name with a dot, this order is reversed. For example, in the documentation of Python’s codecs module, :py:func:`open` always refers to the built-in function, while :py:func:`.open` refers to codecs.open().

Also relevant when using the "." prefix:

> Since this can get ambiguous, if there is more than one possible match, you will get a warning from Sphinx.

but I'm guessing warnings are ignored in the CI docs build (since I see warnings on my machine and build passes). > Since this can get ambiguous, if there is more than one possible match, you will get a warning from Sphinx.

Normally we build the docs with the option to turn warnings into errors on CI (now, there are a bunch of warnings, but those are from the ipython directive, and not done through sphinx' warning machinery, so not causing errors). In any case, on CI here there are indeed no warnings. So given that it should give warnings in case of a possible duplicate, I am fine with using this `.` syntax (at least for the classes like groupby related which are quite nested) @jorisvandenbossche I'm having 2nd thoughts on my last two commits. Adding the `.` unnecessarily and removing any non-necessary `()` is making the diff here significantly larger than it needs to be with no change in the doc output. Is the consistency in the reST worth it? going to merge this on green.  thanks @rhshadrach 
not sure what's up with the windows failure.