Fix up some documentation for RB Gateway.
Fix up some documentation for RB Gateway.

The new RB Gateway rewrite had some incorrect docs in places, detailing
wrong return values and missing exceptions. This fixes the cases I ran
across.

Testing Done:
Checked the docs for issues.

Reviewed at https://reviews.reviewboard.org/r/10638/

reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard