Update documentation to reflect ExactlyOnceDelivery for PostgreSQL Pub/Sub

[thirumal-rafay]

I see that this PR added support for ExactlyOnceDelivery for Postgres, but the documentation may not have been updated to reflect this.

Also, I was reading your tests, and noticed a strange thing at this line. What is special about the Pgx driver where it cannot guarantee ExactlyOnce delivery? Maybe document that fact as well to indicate partial support.

func TestPgxPostgreSQLPublishSubscribe(t *testing.T) {
	t.Parallel()

	features := tests.Features{
		ConsumerGroups:      true,
		ExactlyOnceDelivery: false,
		GuaranteedOrder:     true,
		Persistent:          true,
	}