Clarify target platforms of winrt port

Building desktop applications using the winrt port might be possible, but
it is by far not the main reason for having this port. Stating that we
support desktop builds using Qt for WinRT just causes confusion.

Instead we further clarify the term UWP in a short sentence inside the
winrt part of "supported platforms".

Change-Id: I7d1762f5b450dab74ff3c4e1e1aa16586f242319
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

doc/src/platforms/supported-platforms.qdocinc

@@ -89,9 +89,6 @@
         \li \l{https://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win32/Personal%20Builds/mingw-builds/7.3.0/threads-posix/dwarf/i686-7.3.0-release-posix-dwarf-rt_v5-rev0.7z/download}{MinGW-builds GCC 7.3.0 (x86)}
 \endtable
 
-\br
-\l{Qt for WinRT} can also be used to build applications for Window desktop platforms.
-
 //! [windows]
 
 //! [desktop]
@@ -138,6 +135,11 @@
         \li \b{MSVC 2017}, \b{MSVC 2015}
         \li \b{Windows 10} Creators Update
 \endtable
+
+\br
+Universal Windows Platforms include Windows 10, Windows 10 IoT devices, Xbox
+One, and HoloLens.
+
 //! [winrt]
 
 //! [mobile]