pgsql: docs: clarify the interaction of clientcert and cert auth.

docs: clarify the interaction of clientcert and cert auth. This is the first paragraph change of master-only commit 253f1025da. Backpatch-through: PG 12-13 only Branch -- REL_13_STABLE Details --- https://git.postgresql.org/pg/commitdiff/ef40ab77d5143385d15dcfd08c5a7d66719ef7a3 Modifi

pgsql: docs: clarify the interaction of clientcert and cert auth.

docs: clarify the interaction of clientcert and cert auth. This is the first paragraph change of master-only commit 253f1025da. Backpatch-through: PG 12-13 only Branch -- REL_12_STABLE Details --- https://git.postgresql.org/pg/commitdiff/f05ca47132ddf58de463fd8396a44d7ccd06e123 Modifi