DOC: Add examples to .get_loc methods
- [ x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This pull request adds some examples to ``.get_loc`` doc string of ``pd.Index``, ``pd.CategoricalIndex``, ``pd.MultiIndex`` & ``pd.IntervalIndex``. Also clarifies the return value in some cases.

I previously proposed pull request #17380, but the commit history there became a mess. I've deleted that pull request and added this instead.