ALSA: seq: add documentation for snd_seq_kernel_client_ctl

This kernel API is used by kernel implementation. Currently, it's used for
kernel clients of ALSA sequencer, while it can be used for application
clients. The difference is just on address spaces of argument. In short,
this kernel API can be available for application client with data in kernel
space.

This commit adds a document about this.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_clientmgr.c

@@ -2423,9 +2423,17 @@ int snd_seq_kernel_client_dispatch(int client, struct snd_seq_event * ev,
 
 EXPORT_SYMBOL(snd_seq_kernel_client_dispatch);
 
-/*
- * exported, called by kernel clients to perform same functions as with
- * userland ioctl() 
+/**
+ * snd_seq_kernel_client_ctl - operate a command for a client with data in
+ *			       kernel space.
+ * @clientid:	A numerical ID for a client.
+ * @cmd:	An ioctl(2) command for ALSA sequencer operation.
+ * @arg:	A pointer to data in kernel space.
+ *
+ * Against its name, both kernel/application client can be handled by this
+ * kernel API. A pointer of 'arg' argument should be in kernel space.
+ *
+ * Return: 0 at success. Negative error code at failure.
  */
 int snd_seq_kernel_client_ctl(int clientid, unsigned int cmd, void *arg)
 {