Add documentation for manual proxy settings.

Change-Id: I5661452428ad0918836ab8c9c28f317ed4c1f391
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627551
Commit-Queue: Eric Roman <eroman@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663288}
2 files changed