API: Add doc warning to ApiPageSet::populateFromQueryResult

The cause of bug 72764 was an extension misusing
ApiPageSet::populateFromQueryResult(). Add a warning in the docs for
that method about proper usage.

Bug: 72764
Change-Id: I5b0a069d796f7f41ab0b16048d7b3d6600207383
(cherry picked from commit 25a6a8669037b617a25c36ca43d52b383538e3a2)