DOC: further clarifications to docstring guide
I merged the docstring guide https://github.com/pandas-dev/pandas/pull/19704 in its current state from the sprint.

But based on the sprint experience, I think we noticed a few areas that can use further clarifications to have less confusion / discussion:

- Better overview of when to use which type of quotes (single quotes `'` or single `` ` `` or double backticks ``` `` ```). The way I currently review:
  - no quotes in type description (even if it may need one according to next rules)
  - single backticks to reference parameter names
  - double backticks for small code snippets, like `False` or `param=value`, but, often used class names of pandas like Series, DataFrame, Index are not quoted.
- In specifications like "See Also" or "Returns", include `.pandas` or not?
- Do we use refer to missing values as "NA", "NaN", "NA/NaN", ..
- Are we OK with using "label" in type descriptions to refer to column names? (instead of `str`, as this is in principle too restrictive)
- ...

Did you experience any others?

Let's discuss here what to do with them. Detailed inline comments can still be made on https://github.com/pandas-dev/pandas/pull/19704

cc @datapythonista @WillAyd @TomAugspurger @jreback 