QCollator: Add note about ICU dependency.

Task-number: QTBUG-28766
Change-Id: If661915457c4874a72e1111b85897aea596362bf
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qcollator.cpp

@@ -205,6 +205,11 @@ QLocale QCollator::locale() const
 
     By default this mode is off.
 
+    \note On Windows, this functionality makes use of the \l{ICU} library. If Qt was
+    compiled without ICU support, it falls back to code using native Windows API,
+    which only works from Windows 7 onwards. On older versions of Windows, it will not work
+    and a warning will be emitted at runtime.
+
     \sa numericMode()
  */