Documentation: fix description of Q_OS_WIN32 and add Q_OS_WIN

The documentation for Q_OS_WIN32 states that it is "Defined on all supported versions of Windows.", which seems to be false (not defined for Windows CE, according to the quick look that I had to the sources).

The macro which is defined for all versions of Windows is Q_OS_WIN, which is not documented.

Moreover, it would be more consistent to define Q_OS_WIN32 only for 32-bits Windows, since there's Q_OS_WIN that already gathers all versions of Windows.