Add documentation to goto_trace_constant_pointer_exprt

Author: smowton

src/goto-programs/goto_trace.h

@@ -293,7 +293,11 @@ void show_goto_trace(
     options.set_option("stack-trace", true);
 
 /// Variety of constant expression only used in the context of a GOTO trace, to
-/// give both the numeric value and the symbolic value of a pointer.
+/// give both the numeric value and the symbolic value of a pointer,
+/// e.g. numeric value "0xabcd0004" but symbolic value "&some_object + 4". The
+/// numeric value is stored in the `constant_exprt`'s usual value slot (see
+/// \ref constant_exprt::get_value) and the symbolic value is accessed using the
+/// `symbolic_pointer` method introduced by this class.
 class goto_trace_constant_pointer_exprt : public constant_exprt
 {
 public: