QT_DLL, QT_MAKEDLL and QT_SHARED macros need proper documentation

QT_DLL, QT_MAKEDLL and QT_SHARED in qwebkitglobal.h and phonon_export.h

It seems there some confusion over the meaning of these macros (particularly QT_DLL vs QT_SHARED) so this attempts to document what we did and why

Easy one first - QT_MAKEDLL - this means you're building the Qt libraries themselves, as DLLs (ie static)

QT_DLL - means that the Qt libraries were built as DLLs, rather than statically; but the important question is - is this a subset of QT_SHARED, an alternative to QT_SHARED on platforms that use DLLs rather than shared objects or something else? We opted for considering it a alternative to QT_SHARED in the header files above.

QT_SHARED - reverse set of questions to QT_DLL, again we've opted to understand it as shared object, rather than DLL-based, platforms.