More doc edits (#19929)
Summary:
* document `torch.jit.Attribute`
* add JIT one-liner to `README.md`
* misc clarity edits](https://our.intern.facebook.com/intern/diff/15152418/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19929
Pulled By: driazati
Differential Revision: D15152418
fbshipit-source-id: dfee03f0a17300aaf453fcf17f418463288f66c2
