Explicitly show that "escape" closes dialogs in Phabricator

Summary: See T382. I think bill also asked about this (T308); there's no way to
figure it out right now.
Test Plan:

• Hit "?", verified the dialog now reveals that escape closes it.
• Hit escape, dialog vanished.

Reviewed By: jungejason
Reviewers: fzamore, bill, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 763