R1:b8fe991ba28e

Improve description text in the "Create Diff" form

Summary: The textarea is, in fact, above the description!

Test Plan: Description text changed.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D20092