State that external objects must not depend on themselves.

Closes modelica#2845

chapters/functions.tex

@@ -2470,8 +2470,9 @@ \subsection{External Objects}\label{external-objects}
   \lstinline!ExternalObject!. It is not legal to call explicitly the \lstinline!constructor! and
   \lstinline!destructor! functions. The constructor shall initialize the object, and
   must not require any other calls to be made for the initialization to
-  be complete (e.g., from an initial algorithm or initial equation). The
-  destructor shall delete the object, and must not require any other
+  be complete (e.g., from an initial algorithm or initial equation).
+  The arguments to the constructor must not – directly nor indirectly – depend on the external object being constructed.
+  The destructor shall delete the object, and must not require any other
   calls to be made for the deletion to be complete (e.g., from a \lstinline!when terminal()! clause). The constructor shall not assume that pointers sent
   to the external object will remain valid for the life-time of the external object.  An exception is that if the pointer to another external object is
   given as argument to the constructor, that pointer will remain valid as long as the other external object lives.