Add new permission documentation.

go/add-new-permission

Bug: 1382411
Change-Id: I6af16a1353cfdfcc6aa4410ae9acd5161a0ce49d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4013305


Reviewed-by: Ravjit Uppal <ravjit@chromium.org>
Commit-Queue: Illia Klimov <elklm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1071555}