Make "arc land" prompt on "Changes Planned" revisions more explicit

Summary: Fixes T10233. See PHI231. Users sometimes believe this warning is a bug and/or don't understand how they're supposed to resolve it.

Test Plan: Ran arc land on a revision in "Changes Planned", got a sensible prompt. Ran arc land on a revision in another non-accepted state, got more or less the old prompt.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T10233

Differential Revision: https://secure.phabricator.com/D18807