Document wswap_i64(), added in commit 46be8425
("tcg: Implement tcg_gen_{h,w}swap_{i32,i64}").

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230823145542.79633-8-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>