char: add qemu_chr_fe_add_watch() Returns description

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/sysemu/char.h

@@ -258,6 +258,8 @@ void qemu_chr_fe_printf(CharBackend *be, const char *fmt, ...)
  * @cond the condition to poll for
  * @func the function to call when the condition happens
  * @user_data the opaque pointer to pass to @func
+ *
+ * Returns: the source tag
  */
 guint qemu_chr_fe_add_watch(CharBackend *be, GIOCondition cond,
                             GIOFunc func, void *user_data);