Document the new "only if this didn't match last time" Herald action setting
Summary: Depends on D18930. Ref T13048. Try to explain what this does and give an example since I think it's probably not very obvious from the name.
Test Plan: Read the text.
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T13048
Differential Revision: https://secure.phabricator.com/D18931