Improve some docs on rbt post, and add anchors.
Improve some docs on rbt post, and add anchors.

This makes a couple small improvements to the documentation on rbt post,
preferring rbt post -u over -r, and removing the (outdated)
documentation for -g.

It also adds a bunch of anchors we can use to link to usage
documentation for different SCMs. Future changes will build upon this by
splitting out the pages into per-SCM pages.

Testing Done:
Built the docs and tested the references.

Reviewed at https://reviews.reviewboard.org/r/8730/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools