Update comments on removing fields.

Indicate the last time that (now) reserved fields were used, and update
comments on when fields can be made 'reserved', based on what release
branches would be affected.

BUG=chromium:1126855
TEST=generate

Change-Id: Ica1219ed1a2a33ce652863a648b00283c2447bd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2664728
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>
12 files changed