Add informal "noparent" comment to sync/protocol/OWNERS

Bug: none
Change-Id: Ifddb5f627e6334333e3d501e47c2d60789b8c5a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2116210Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752770}

components/sync/protocol/OWNERS

+# Note: Please use only owners from this file for changes to the Sync protos.
+#       Don't use owners from higher-level folders, and *never* TBR changes!
 markusheintz@chromium.org
 mastiz@chromium.org
 treib@chromium.org