R1:dd8232b766ae

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…