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