Hint 'arc' commands in Differential UI

Summary:
Point users toward 'arc amend', 'arc commit', 'arc patch' and 'arc export' since
no one is going to read 'arc help'.

There's kind of a tradeoff here where we're wasting a fair amount of UI space
for expert users with the patch/export hints but I think it's probably okay
since there's really no other way to figure out that these features exist.

Note that the "export" command given isn't complete (it needs --git or
--unified), but it will give you a useful error message when you run it, telling
you to specify --git or --unified. If it turns out users get confused by this,
let me know.

Test Plan:
Loaded a revision and looked at it. Faked it into 'accepted' status.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, aran, jungejason
CC: aran, tuomaspelkonen, epriestley
Differential Revision: 242