Update mathematical-operations.rst

I added a note that the updating-operators can change the type.  This is implicit in the current manual, but it nonetheless threw me for a loop the first time it happened to me (this is different, e.g., from C++), so I thought it should be made explicit.

doc/manual/mathematical-operations.rst

@@ -122,6 +122,17 @@ are::
  += -= *= /= \= %= ^= &= |= $= >>>= >>= <<=
 
 
+It should be noted that updating operators can change the type of the variable. For example, after the first 
+statement below, ``x`` is of type ``Uint32``, but after the second statement, it is of type ``Int``::
+
+ 
+ x = 0x00000001
+ x *= 2
+ 
+ 
+This is in consistent with the equivalence between ``x *= 2`` and ``x = x * 2``: any assignment statement to
+a plain variable may change its type.
+
 .. _man-numeric-comparisons:
 
 Numeric Comparisons