Update Rows.Scan documentation to reflect reality.

Previously, the Scan documentation stated that scanning into a []byte
will skip the decoding process and directly copy the raw bytes received
from PostgreSQL.

This has not been true for at least 2 months. It is also undesirable
behavior in some cases such as a binary formatted jsonb. In that case
the '1' prefix needs to be stripped to have valid JSON. If the raw
bytes are desired this can easily be accomplished by scanning into
pgtype.GenericBinary or using Rows.RawValues.

In light of the fact that the new behavior is superior, and that it has
been in place for a significant amount of time, I have decided to
document the new behavior rather than change back to the old behavior.

values_test.go

@@ -983,3 +983,47 @@ order by a nulls first
 		require.NoError(t, rows.Err())
 	})
 }
+
+func TestScanIntoByteSlice(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnectString(t, os.Getenv("PGX_TEST_DATABASE"))
+	defer closeConn(t, conn)
+	// Success cases
+	for _, tt := range []struct {
+		name             string
+		sql              string
+		resultFormatCode int16
+		output           []byte
+	}{
+		{"int - text", "select 42", pgx.TextFormatCode, []byte("42")},
+		{"text - text", "select 'hi'", pgx.TextFormatCode, []byte("hi")},
+		{"text - binary", "select 'hi'", pgx.BinaryFormatCode, []byte("hi")},
+		{"json - text", "select '{}'::json", pgx.TextFormatCode, []byte("{}")},
+		{"json - binary", "select '{}'::json", pgx.BinaryFormatCode, []byte("{}")},
+		{"jsonb - text", "select '{}'::jsonb", pgx.TextFormatCode, []byte("{}")},
+		{"jsonb - binary", "select '{}'::jsonb", pgx.BinaryFormatCode, []byte("{}")},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			var buf []byte
+			err := conn.QueryRow(context.Background(), tt.sql, pgx.QueryResultFormats{tt.resultFormatCode}).Scan(&buf)
+			require.NoError(t, err)
+			require.Equal(t, tt.output, buf)
+		})
+	}
+
+	// Failure cases
+	for _, tt := range []struct {
+		name string
+		sql  string
+		err  string
+	}{
+		{"int binary", "select 42", "can't scan into dest[0]: cannot assign 42 into *[]uint8"},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			var buf []byte
+			err := conn.QueryRow(context.Background(), tt.sql, pgx.QueryResultFormats{pgx.BinaryFormatCode}).Scan(&buf)
+			require.EqualError(t, err, tt.err)
+		})
+	}
+}