Qt for VxWorks documentation improvements

http://doc.qt.io/qt-5/vxworks.html

To improve the Qt development experience, the followings things should be mentioned in the article:

1) On Windows, Qt for VxWorks should be built with MinGW 5.3.0 installed via Qt Installer.
I ran into issues while I was using MinGW installed from the official online installer that I downloaded from https://sourceforge.net/projects/mingw/files/ and MinGW 4.x installed with Qt.

2) VSB must be built  with the "Qt prerequisite" (Source Build Configuration -> ui -> Enable Raster library 1.0.02 -> Enable Qt prerequisite 1.0.0.0) layer.

3) Qt for VxWorks is confirmed to build normally on Windows 7 and 10 64-bit (and other Windows versions that have been tested).
4) WIND_BASE must be set (by executing

wrenv -p vxworks-7

command) in the calling environment of Qt Creator, otherwise Qt for VxWorks kit won't work.

See also QTCREATORBUG-19177.