CLN: auto-inherit specified Index method docstrings
When working on indexing methods, I find myself grepping for the names, and having the extra Appender lines is a small-but-repeated nuisance.

In py36+ we can use `__init_subclass_` to automatically inherit docstrings without needing to repeat these Appenders.

This PR only does this for a few methods as a proof of concept, but we could expand this list quite a bit.

Based on a handful of measurements, no obvious impact on import time.

cc @WillAyd you were -1 on the proof of concept when it was on PandasObject and automatic for all methods; are you more amenable to this more finely-tuned approach?
I'd prefer to stick with what @HH-MWB has done with the doc decorator for the time being, which combines Appender and Subsitution together > decorator for the time being, which combines Appender and Subsitution together

I think these are addressing different problems.  This is addressing (in the order I _should_ have started with):

1) inconsistency in which methods we inherit docstrings for.  e.g.  `memory_usage` we inherit for `IntervalIndex`, copy/paste (i think, looks a little out of date) for `RangeIndex`, and no docstring for `PeriodIndex`.  Similar for `_maybe_cast_slice_bound` where `IntervalIndex` has no docstring.
2) verbosity that happens to be annoying in my workflow
 mothballing to clear the queue