89734301 introduced a new useful feature
to `git upstream-diff`, but unfortunately also regressed the behavior
of the tool when used with additional arguments for `git diff`.

This adds some additional documentation to demonstrate the intended
original feature (and fixes some of the bit-rot in the documentation
pipeline).

R=agable@chromium.org, mattm@chromium.org

Change-Id: I3ae48db3232c1ac84a7edbfe2225a17cda391a1e
Reviewed-on: https://chromium-review.googlesource.com/1107491
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>