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`)