R1:033e4d999790

Document final/private policy

Summary: Provide explicit guidance in the documentation about liberal use of
"final".

Test Plan: Generated, read documentation.

Reviewers: btrahan, jungejason, aran, nh

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T795

Differential Revision: https://secure.phabricator.com/D1520