R1:19c840edf28c

Better explain 'h' keyboard shortcut in Differential

Summary: Also move 'f' outside this group of commands.

Test Plan: Pressed '?'.

Reviewers: epriestley, alanh, frantic

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3903