ALSA: doc: Update description for the new PCM copy ops

Update the documentation about the PCM copy callbacks.
The update was kept minimalistic, just correcting the use of copy_user
ops with the single copy ops, and drop/update the text mentioning the
copy_kernel.

Link: https://lore.kernel.org/r/20230815190136.8987-24-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>