alsa-lib: Reword doxygen doc for snd_pcm_delay()

alsa-lib: Reword doxygen doc for snd_pcm_delay()

snd_pcm_delay() is for synchronization purposes, it returns the overall latency
of the stream, not just the latency induced by the hardware playback buffer.
The documentation is a bit misleading about this fact, and some people already
misunderstood it. So let's reword this to make the explanation clearer and
explicit.

This is a result of the discussions of the thread this mail belongs to:

http://mailman.alsa-project.org/pipermail/alsa-devel/2008-June/008456.html

From: Lennart Poettering <mznyfn@0pointer.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>