Document platform limitations for QCollator::ignorePunctuation

OS X and iOS do have a flag for this in their API, but it does
not seem to work. We can't support this on Linux without ICU.

Change-Id: I81613ad425cb054597f23fac112be665f8d958b6
Task-number: QTBUG-41978
Reviewed-by: Jake Petroules <jake.petroules@theqtcompany.com>

src/corelib/tools/qcollator.cpp

@@ -248,6 +248,8 @@ bool QCollator::numericMode() const
 
     The default is locale dependent.
 
+    \note This method is not currently supported on Apple platforms or if Qt is configured to not use ICU on Linux.
+
     \sa ignorePunctuation()
  */
 void QCollator::setIgnorePunctuation(bool on)