Summary:
Fixes T5532. Allow documents to have a preamble in the header which can be used to explain who should sign a document and why.
Particularly, I plan to use this to navigate the corporate vs individual stuff more sensibly.
Test Plan: {F174228}
Reviewers: chad
Reviewed By: chad
Subscribers: epriestley
Maniphest Tasks: T5532
Differential Revision: https://secure.phabricator.com/D9819
