[docs] Add updated TorchScript language reference section for types (#53673)
Summary:
**Summary**
This commit adds information about type annotation and inference to
the updated language specification. It will be rebased on top of https://github.com/pytorch/pytorch/issues/52494
after it lands.
**Test Plan**
Continuous integration.
Screen capture:
https://user-images.githubusercontent.com/4392003/110560184-66371f80-80fa-11eb-803a-923cf8de25ff.mov
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53673
Reviewed By: gmagogsfm
Differential Revision: D27413001
Pulled By: SplitInfinity
fbshipit-source-id: b54b300b4b1f10537ec06e2ee9eeb6d2b1f1810b
