Fix CoMarshalInterThreadInterfaceInStream

[Robert Shearman]

Hi,

This patch fixes CoMarshalInterThreadInterfaceInStream and
CoGetInterfaceAndReleaseStream to match what MSDN says - calling
CoMarshalInterface and CoUnmarshalInterface respectively. Without this
patch a particular InstallShield installer would just hang. With this
patch it gets a bit further (to an unimplemented function).
I also took the liberty of writing the documentation for both the
functions while I was there.

Rob

Changelog:
Fix CoMarshalInterThreadInterfaceInStream and
CoGetInterfaceAndReleaseStream.