arc: document proxy bypass list browser test

This CL is based on comments on CL:1630481.

Bug: b:133193788
Change-Id: I67abc6b31dae67a58c77b555d0628dcb1f610799
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642728
Commit-Queue: Andreea-Elena Costinas <acostinas@google.com>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666656}