hhvm manual TOC, some basic jit docs

Summary:
I had a lot of trouble trying to decide what granularity we want in
the TOC, but it's best to just get something in and then iterate as we fill it
out. This also includes some docs on region selection and a couple
optimizations. It's written for the (hopefully) near future where
Translator::analyze has been replaced by the region selector.

Test Plan: read it

Reviewers: smith, jdelong, bertrand, ottoni, je, oyamauchi, mwilliams

CC: smith, ps, hphp-diffs@lists

Differential Revision: https://phabricator.fb.com/D1191045

Task ID: 3771853