Enhance the documents about the PCM, missing descriptions for a couple
of helpers like snd_pcm_period_elapsed_under_stream_lock() and
snd_pcm_stop_xrun().

Link: https://lore.kernel.org/r/20230323065237.5062-4-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>