Improve help and prompts for the "--draft" flag

Summary:
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.

Test Plan:

• 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