Improve documentation for BaseReviewResource.update_review().
Improve documentation for BaseReviewResource.update_review().

`BaseReviewResource.update_review()` takes several important arguments,
which weren't previously documented. This is a simple change that
documents the function a bit better, adding information on the arguments
and return type.

Testing Done:
Unit tests passed.

Reviewed at https://reviews.reviewboard.org/r/9304/

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