hswish and hardsigmoid: improve docs (#35431)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35431
Resolving z-a-f's comments on earlier PRs on making
the docblocks easier to read.
Test Plan:
render the new docblocks in http://rst.aaroniles.net/
CI
Imported from OSS
Differential Revision: D20658668
fbshipit-source-id: 5ea4a21d6b8dc9d744e2f4ede2f9d5d799fb902f
