Explain the relationship between "Runnable" and "Restartable" more clearly in Build Plans

Summary: See PHI1153. The "Runnable" and "Restartable" behaviors interact (to click "restart", you must be able to run the build AND it must be restartable). Make this more clear.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D20307