Unregistered OIDs are handled the same as unknown OIDs

This improves handling of unregistered types. In general, they should
"just work". But there are performance benefits gained and some edge
cases avoided by registering types. Updated documentation to mention
this.

https://github.com/jackc/pgx/issues/1445

query_test.go

@@ -190,7 +190,7 @@ func TestConnQueryValuesWhenUnableToDecode(t *testing.T) {
 	require.Equal(t, "({1},)", values[0])
 }
 
-func TestConnQueryValuesWithUnknownOID(t *testing.T) {
+func TestConnQueryValuesWithUnregisteredOID(t *testing.T) {
 	t.Parallel()
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
@@ -217,6 +217,24 @@ func TestConnQueryValuesWithUnknownOID(t *testing.T) {
 	require.Equal(t, "orange", values[0])
 }
 
+func TestConnQueryArgsAndScanWithUnregisteredOID(t *testing.T) {
+	t.Parallel()
+
+	pgxtest.RunWithQueryExecModes(context.Background(), t, defaultConnTestRunner, nil, func(ctx context.Context, t testing.TB, conn *pgx.Conn) {
+		tx, err := conn.Begin(ctx)
+		require.NoError(t, err)
+		defer tx.Rollback(ctx)
+
+		_, err = tx.Exec(ctx, "create type fruit as enum('orange', 'apple', 'pear')")
+		require.NoError(t, err)
+
+		var result string
+		err = conn.QueryRow(ctx, "select $1::fruit", "orange").Scan(&result)
+		require.NoError(t, err)
+		require.Equal(t, "orange", result)
+	})
+}
+
 // https://github.com/jackc/pgx/issues/478
 func TestConnQueryReadRowMultipleTimes(t *testing.T) {
 	t.Parallel()