C++: simplify PointerOverflow.qhelp

Author: Robert Marsh

cpp/ql/src/Likely Bugs/Memory Management/PointerOverflow.qhelp

@@ -4,19 +4,17 @@
 <qhelp>
 <overview>
 <p>
-The expression <code>ptr + a &lt; ptr</code> is equivalent to <code>a &lt;
-0</code>, and an optimizing compiler is likely to make that replacement,
-thereby removing a range check that might have been necessary for security.
-If <code>a</code> is known to be non-negative, the compiler can even replace <code>ptr +
-a &lt; ptr</code> with <code>false</code>.
+When checking for integer overflow, you may often write tests like
+<code>a + b &lt; a</code>.  This works fine if <code>a</code> and
+<code>b</code> are unsigned integers, since any overflow in the addition
+will cause the value to simply "wrap around."  However, using this pattern when
+<code>a</code> is a pointer is problematic because pointer overflow has
+undefined behavior according to the C and C++ standards.  If the addition
+overflows and has an undefined result, the comparison will likewise be
+undefined; it may produce an unintended result, or may be deleted entirely by an
+optimizing compiler.
 </p>
 
-<p>
-The reason is that pointer arithmetic overflow in C/C++ is undefined
-behavior. The optimizing compiler can assume that the program has no
-undefined behavior, which means that adding a positive number to <code>ptr</code> cannot
-produce a pointer less than  <code>ptr</code>.
-</p>
 </overview>
 <recommendation>
 <p>