Doc: You need to use x64_x86 cross-compiler to compile for 32 bit

Task-number: QTBUG-68462
Change-Id: I6d3358d36bb3df91c05d434275e9c69682c982a9
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>

src/webengine/doc/src/qtwebengine-platform-notes.qdoc

@@ -71,6 +71,12 @@
 
     On Windows, Visual Studio 2017 and Windows 10 SDK are required.
 
+    Qt WebEngine can only be built on 64-bit Windows, with a x64-bit toolchain.
+    For building Qt WebEngine for x86 applications, you need to configure
+    and compile Qt with the Visual Studio 2017 x64 to x86 cross-compile
+    toolchain. This toolchain can be set up on the command line by running
+    \c{vcvarsall.bat amd64_x86}.
+
     \section2 Linux
 
     On Linux, Clang or GCC version 5 or later is required.