gss: add missing references to hostgssenc and hostnogssenc
These were missed when these were added to pg_hba.conf in PG 12;
updates docs and pg_hba.conf.sample.
Reported-by: Arthur Nascimento
Bug: 16380
Discussion: https://postgr.es/m/
20200421182736.GG19613@momjian.us
Backpatch-through: 12