gh-94590: add signatures to operator itemgetter, attrgetter, methodcaller (#94591)

These were intentionally skipped when operator was updated to use the argument clinic:
https://github.com/python/cpython/issues/64385#issuecomment-1093641466

However, by not using the argument clinic, they missed out on getting signatures.
This is a narrow PR to update the docstrings so that `__text_signature__` can be
extracted from them.  Updating to use the argument clinic is beyond scope.

`methodcaller` uses `*args, **kwargs` to match variadic names used elsewhere,
including in `operator.call`.