[expr.add] Move note, add example (#4778)

The original note wasn't quite in the right place.
It is moved and made more concrete.

In the original place of the note we now put
a more immediately relevant example.

source/expressions.tex

@@ -6234,6 +6234,11 @@
 $i - j$ of \tcode{x} if $0 \le i - j \le n$.
 \item Otherwise, the behavior is undefined.
 \end{itemize}
+\begin{note}
+Adding a value other than $0$ or $1$
+to a pointer to a base class subobject, a member subobject,
+or a complete object results in undefined behavior.
+\end{note}
 
 \pnum
 \indextext{\idxcode{ptrdiff_t}!implementation-defined type of}%
@@ -6265,10 +6270,14 @@
 For addition or subtraction, if the expressions \tcode{P} or \tcode{Q} have
 type ``pointer to \cv{}~\tcode{T}'', where \tcode{T} and the array element type
 are not similar\iref{conv.qual}, the behavior is undefined.
-\begin{note}
-In particular, a pointer to a base class cannot be used for
-pointer arithmetic when the array contains objects of a derived class type.
-\end{note}
+\begin{example}
+\begin{codeblock}
+int arr[5] = {1, 2, 3, 4, 5};
+unsigned int *p = reinterpret_cast<unsigned int*>(arr + 1);
+unsigned int k = *p;            // OK, value of \tcode{k} is $2$\iref{conv.lval}
+unsigned int *q = p + 1;        // undefined behavior: \tcode{p} points to an \tcode{int}, not an \tcode{unsigned int} object
+\end{codeblock}
+\end{example}
 
 \rSec2[expr.shift]{Shift operators}