VS: Do not compile C sources as WinRT (#15100)

The MSVC /ZW flag is valid only for C++ sources.  Whenever we enable
CompileAsWinRT for the whole target, disable it for all C sources.
Update the documentation of VS_WINRT_COMPONENT to drop the statement
about undefined behavior for non-C++ sources, because it is now
defined as expected.
Help: Add notes for topic 'vs-windows-apps'
Merge topic 'vs-windows-apps' into next

e30e4a83 Help: Add notes for topic 'vs-windows-apps'
97a90062 VS: Do not compile C sources as WinRT (#15100)