[Nearby] Add documentation for why bandwidth upgrade is delayed.

Change-Id: Ic7069e708349635c0200cbe1d5740bdbfc169a31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2667534


Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: James Vecore <vecore@google.com>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#850441}