seq: improve documentation about new get pid/card functions

Document the technique for determining if the running kernel supports
the new snd_seq_client_info_get_pid and snd_seq_client_info_get_card
functions. Also add a little information about how to use these
functions and add some cross references.

Signed-off-by: Adam Goode <agoode@google.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>