axfer: add a manual for transfer subcommand

axfer: add a manual for transfer subcommand

This commit adds a manual for transfer subcommand in axfer(1). This
subcommand is a main feature of this command, to transfer audio data
frame between device/node and file/stdio.

This subcommand is designed to have several transmission backend.
Detail explanation about these backends is added in future commits.

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