Add a comment to placeholder migrations.

We've had a review that tried to reuse migration 162 rather than
skipping to 172.  Maybe a comment will prevent that in the future.

Change-Id: Ib8b73260df593d13a81ce65c85a302a54668b95e
Merge "Add a comment to placeholder migrations."
Merge remote-tracking branch 'm_src_repo/master' into build-temp