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?