pgsql: Improve programmer docs for simplehash and dynahash.

Improve programmer docs for simplehash and dynahash.

When reading the code it's not obvious when one should prefer dynahash
over simplehash and vice-versa, so, for programmer-friendliness, add
comments to inform that decision.

Show sample simplehash method signatures.

Author: James Coleman <jtc331@gmail.com>
Discussion: https://postgr.es/m/CAAaqYe_dOF39gAJ8rL-a3YO3Qo96MHMRQ2whFjK5ZcU6YvMQSA%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/84c0e4b9bce794da914fe9c062753bf21369745f

Modified Files
--------------
src/backend/utils/hash/dynahash.c | 12 ++++++-
src/include/lib/simplehash.h      | 73 ++++++++++++++++++++++++++++++++++++---
2 files changed, 80 insertions(+), 5 deletions(-)