axfer: add a manual for entry point

axfer: add a manual for entry point

This commit adds a manual for axfer(1). This command is an entry point
to supported subcommand.

I note that axfer(1) has renewed option system against aplay(1). The
command get positional two options for subcommand and direction. In
short:
 $ axfer transfer capture|playback [options for transfer subcommand]
 $ axfer list capture|playback [options for list subcommand]
 $ axfer version
 $ axfer help

Manuals to subcommands are added later separately.

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