Document run.scheduleOnce, truncate run.once docs. Fixes #2132.

This is a tradeoff to avoid repeating the docs for scheduleOnce on
once. Most people may intend to use run.once, but scheeduleOnce is
certainly widely enough used to deserve documentation of it's own.
Merge pull request #2499 from mixonic/scheduleOnce-documentation

Document run.scheduleOnce, truncate run.once docs. Fixes #2132.