Doc: add note about unsupported platforms for QProcess

Task-number: QTBUG-57840
Change-Id: I46a26a9c4c6ad0aa6994945091a2904c3b51080f
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

src/corelib/io/qprocess.cpp

@@ -509,6 +509,9 @@ void QProcessPrivate::Channel::clear()
     You can also call error() to find the type of error that occurred
     last, and state() to find the current process state.
 
+    \note QProcess is not supported on VxWorks, iOS, tvOS, watchOS,
+    or the Universal Windows Platform.
+
     \section1 Communicating via Channels
 
     Processes have two predefined output channels: The standard