Improve comments about partitioned hash table freelists.
Improve comments about partitioned hash table freelists.

While I couldn't find any live bugs in commit 44ca4022f, 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: 85fecb2552e3a9b39033ac4d9b7b729eeb6a60ed@sss.pgh.pa.us

50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb