chardev/char-fe: Document returned value on error

qemu_chr_fe_add_watch() and qemu_chr_fe_write[_all]()
return -1 on error. Mention it in the documentation.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20240722160745.67904-2-philmd@linaro.org>
1 file changed