Flesh out the rbt alias documentation.
Flesh out the rbt alias documentation.

This updates the `rbt alias` documentation to be a bit more thorough,
going into detail on the options available and showing more real-world
examples.

Testing Done:
Built the docs. Checked for build errors, spelling errors, and link
errors.

Reviewed at https://reviews.reviewboard.org/r/12196/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools