ALSA: pcm: add a documentation for tracepoints

In PCM interface/protocol for userspace, parameters of runtime for PCM
substream is decided by an interaction between applications and ALSA
PCM core. In former commits, some tracepoints were added to probe a part
of the interaction.

This commit adds a documentation about the interaction and the
tracepoints.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>