Snd: Help text improvements and cleanup.

Reformat and update help texts.

If the PsychPortAudio driver is opened for exclusive use on Linux and
Windows systems, give the users some additional online instructions on
how she should proceed if she wants to use both Snd() and
PsychPortAudio at the same time: First open PsychPortAudio audio
device, then use Snd() et al. - This will make sure Snd() uses the
classic fallback path and therefore doesn't block PsychPortAudio for
use by the users experiment script.