R1:5333b16d7f1d

Clarify header block documentation

Summary: Make it more explicit that headers are block formatters, see T778.

Test Plan: Read docs.

Reviewers: davidreuss, btrahan, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T778

Differential Revision: https://secure.phabricator.com/D1420