[COMMITTERS] pgsql: 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: https://postgr.es/m/10593.1500670709@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ab2324fd468c4bc6d8b012552550ed951d97339a

Modified Files
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src/backend/utils/hash/dynahash.c | 105 +++++++++++++++++++++++++-------------
1 file changed, 70 insertions(+), 35 deletions(-)