Add documentation on rbt close --json.
Add documentation on rbt close --json.

This adds a section to the `rbt close` docs that covers the new `--json`
parameter. This documents the success and error schema, and shows off
usage for both.

Testing Done:
Built the docs. Checked for any obvious errors or spelling errors.

Reviewed at https://reviews.reviewboard.org/r/12168/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools