be more precise with temp_pntrString

Author: wlammen

src/mmdata.h

@@ -160,8 +160,10 @@ typedef nmbrString temp_nmbrString;
  * \brief a single \ref pntrString element for use in a \ref stack "stack".
  *
  * These elements are pushed onto and popped off a \ref stack 
- * "stack of temporary data".  Special commands can free all pointers on and
- * after a particular one in such a stack.
+ * "stack of temporary data".  All pointer of this type should ONLY refer to
+ * adynamically llocated memory on the heap.  Special commands support
+ * dependency tracking and free all pointers on and after a particular one in
+ * such a stack. 
  */
 typedef pntrString temp_pntrString;