PsychVideoSwitcher: Clarify help text for 'enableLuminanceMode'

That parameter is ignored in non-PCI card versions of the VideoSwitcher,
ie. in pretty much all shipping (external) boxes.
Updated by Xiangrui Li.
DownloadPsychtoolbox: Do not abort on 32-Bit Matlab for OSX!

Otherwise downloading the '3.0.10' flavor for 32-Bit OSX would not work.
Fixes #157
help DisplayOutputMappings: Improve help for multi-gpu setups.

Tell user she also needs to set the 'ScreenToHead' mappings if
PsychTweak('UseGPUIndex') is used to select a non-default GPU. Clarify
the limitations.

Hopefully helps for cases like the one described in forum message
#16208.