doc: Clarify OAuth validator authn_id logging

doc: Clarify OAuth validator authn_id logging

Document that OAuth validators can return an authenticated identity
in the authn_id member.  The server records the identity value before
checking if the connection is authorized, so it may appear in
connection-authentication logs (even if the connection later fails
authorization).

Also remove outdated wording saying that all result parameters are
ignored when a validator returns false since validators may provide
error_detail.

Patch by Chao Li with some additional wordsmithing by me.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Reported-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/0281836A-F5FF-41A5-9EE1-656C1FAAC6B2@gmail.com