While I couldn't find any live bugs in commit 44ca4022, the comments
seemed pretty far from adequate; in particular it was not made plain that
"borrowing" entries from other freelists is critical for correctness.
Try to improve the commentary.  A couple of very minor code style
tweaks, as well.

Discussion: https://postgr.es/m/10593.1500670709@sss.pgh.pa.us