Improve help and prompts for the "--draft" flag
See PHI458. The help text and prompting for "arc diff --draft" aren't very clear about whether updating publishes or not, nor about whether you need to keep passing "--draft" every time.
Make these behaviors more clear.
- Ran arc help diff, read text.
- Updated a draft with "--draft", got new warning but it went through.
- Updated a published revision with "--draft", got new error.
Differential Revision: https://secure.phabricator.com/D19229