DOC: Add See Also section to IndexSlice docstring regarding removing unused levels
The behavior described in the [Defined Levels](https://pandas.pydata.org/pandas-docs/stable/advanced.html#defined-levels) section on the docs is often a source of confusion when slicing a `MultiIndex`.

Adding a "See Also" section to the docstring for [`IndexSlice`](https://pandas.pydata.org/pandas-docs/version/0.22/generated/pandas.IndexSlice.html) that includes a link to Defined Levels section could be beneficial in giving this behavior visibility.  A link to [`MultiIndex.remove_unused_levels`](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.MultiIndex.remove_unused_levels.html) in the new "See Also" section might also be helpful.

xref #21299