AW: prefer trailing commas

No change to logic. This adds comments telling contributors to
prefer trailing commas in lists because this allows cleaner diffing
(and, it makes it easier to use git-blame to identify who owns the
particular feature in the list).

This also disables line-length warnings and code formatting for the
KNOWN_MISTAKES dictionary for flag labels, to make it easier for
contributors to add entries.

Test: git cl presubmit --upload
Test: git cl format
Change-Id: I458a654544464e6b4d8975462833472b0d6388d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3552085
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#985290}