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