speaker-test.1: Clarify man page by giving examples

speaker-test.1: Clarify man page by giving examples

BugLink: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=620513
From the Debian bug report:
"The speaker-test manpage could be improved somewhat by helping a
little bit the user determine how to find out the devices he can test.

Based on http://alsa.opensrc.org/Speaker-test I have make some changes
to the manpage (diff attached) to describe a little bit more how
speaker-test can be used alongside 'aplay'.

Please consider the following patch for
alsa-utils-1.0.23/speaker-test/speaker-test.1"

Signed-off-by: Javier Fernández-Sanguino Peña <jfs@debian.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>