Improve/correct comments

Improve/correct comments

Change comments for pg_cryptohash_init(), pg_cryptohash_update(),
pg_cryptohash_final() in cryptohash.c to match cryptohash_openssl.c.
In particular, the claim that these functions were "designed" to never
fail was incorrect, since by design callers need to be prepared to
handle failures, for compatibility with the cryptohash_openssl.c
versions.

Reviewed-by: Daniel Gustafsson 
Discussion: https://www.postgresql.org/message-id/301F4EDD-27B9-460F-B462-B9DB2BDE4ACF@yesql.se