R1:de0261358f61

Document all the SafeHTML stuff

Summary: Write documentation about how to use phutil_tag(), etc., safely.

Test Plan:
Read carefully.

{F31223}

Reviewers: vrana, btrahan, chad

Reviewed By: vrana

CC: aran

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4769