Write more detailed documentation about Differential inlines

Summary: Ref T9628. The porting feature has been fairly stable for a while, so make some reasonable effort to document how it works and some of the tradeoffs it involves.

Test Plan: Generated and read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9628

Differential Revision: https://secure.phabricator.com/D14335