DOC: add guide on shared docstrings
closes https://github.com/pandas-dev/pandas/issues/16446

xref https://github.com/pandas-dev/pandas/issues/19932

cc @jorisvandenbossche 

The main change in best practices here is the recommendation is that `Appender` is only for appending. `Substitution` should be used for substituting values.

See the changes to `fillna` that this policy requires. Personally, I find the new way clearer than the old way.

- We write the base template normally
- I like `Appender(NDFrame.method.__doc__)` better than `Appender(_shared_docs['name'] % kwargs)`
- I like `Substitution(**kwargs)` better than `%` formatting.

The downsides is that we don't have the template in a dictionary anymore and the docstring in `NDFrame` has to be the template, it can't have its variables substituted. I think that's OK.