Fix setrefs.c comment properly.

The "typo" alleged in commit 1e460d4b was actually a comment that was
correct when written, but I missed updating it in commit b5282aa8.
Use a slightly less specific (and hopefully more future-proof) description
of what is collected.  Back-patch to 9.2 where that commit appeared, and
revert the comment to its then-entirely-correct state before that.