R1:25ffde6c5f09

Update Herald docs

Summary:
Update the Herald documentation with information about global vs personal rules.

NOTE: Some of the features mentioned here don't exist yet (flags, delete rules), I'll add them before I land this.

Test Plan: Read documentation

Reviewers: jungejason, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2031