GitLab

- Remove an extra heading under "Rebaselining using try jobs" >
  "Options".
- Add doc for '--patchset=n'.
- Add link to local manual rebaselining.

TBR=robertma@chromium.org

Change-Id: I54d1daf6754707009c08cbc53791a70262bca206
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2005408
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732573}