Updated documentation for rbt stamp
Updated documentation for rbt stamp

- Explained the options in more detail
- Mentioned the ability to stamp from `rbt post`

Testing Done:
Built the HTML and things look reasonable.

Reviewed at https://reviews.reviewboard.org/r/7321/

beol/rbtools,davidt/rbtools,haosdent/rbtools,beol/rbtools,datjwu/rbtools,beol/rbtools,datjwu/rbtools,davidt/rbtools,datjwu/rbtools,reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools,haosdent/rbtools,haosdent/rbtools,davidt/rbtools