Update wrong docs for TouchPoint::id().

What it states is simply not true, at least certainly
not since QPA has been introduced.

Change-Id: If8aa2120cf5fc9ca85f4f0e6832b2115a5b71fe1
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/gui/kernel/qevent.cpp

@@ -3567,9 +3567,8 @@ QTouchEvent::TouchPoint::~TouchPoint()
 /*!
     Returns the id number of this touch point.
 
-    Id numbers are globally sequential, starting at zero, meaning the
-    first touch point in the application has id 0, the second has id 1,
-    and so on.
+    Do not assume that id numbers start at zero or that they are sequential.
+    Such an assumption is often false due to the way the underlying drivers work.
 */
 int QTouchEvent::TouchPoint::id() const
 {