[JIT] improve documentation (#57991)
Summary:
* Fix lots of links.
* Minor improvements for consistency, clarity or grammar.
* Update jit_python_reference to note the limitations on __exit__.
(Related to https://github.com/pytorch/pytorch/issues/41420).
* Fix a comment in exit_transforms.cpp: removed the word "not" which
made the comment say the opposite of the truth.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57991
Reviewed By: malfet
Differential Revision: D28522247
Pulled By: SplitInfinity
fbshipit-source-id: fc63a59d19ea6c89f957c9f7d451be17d1c5fc91
