ALSA: document that struct __snd_pcm_mmap_control64 is messed up

I'm not the first one to run into this, see e.g.
https://lore.kernel.org/all/29QBMJU8DE71E.2YZSH8IHT5HMH@mforney.org/

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230406132521.2252019-1-oswald.buddenhagen@gmx.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/uapi/sound/asound.h

@@ -570,7 +570,8 @@ struct __snd_pcm_mmap_status64 {
 struct __snd_pcm_mmap_control64 {
 	__pad_before_uframe __pad1;
 	snd_pcm_uframes_t appl_ptr;	 /* RW: appl ptr (0...boundary-1) */
-	__pad_before_uframe __pad2;
+	__pad_before_uframe __pad2;	 // This should be __pad_after_uframe, but binary
+					 // backwards compatibility constraints prevent a fix.
 
 	__pad_before_uframe __pad3;
 	snd_pcm_uframes_t  avail_min;	 /* RW: min available frames for wakeup */