Update symbols docs

Summary: Update manual to reflect T7984 and T8302.

Test Plan: Not many new red squigly lines.

Reviewers: joshuaspence, epriestley, Blessed Reviewers

Reviewed By: joshuaspence, epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13551