Document that PyString_FromString() cannot take NULL as a parameter.

This closes SF bug #489872.

Doc/api/concrete.tex

@@ -463,7 +463,8 @@ parameter and are called with a non-string parameter.
 
 \begin{cfuncdesc}{PyObject*}{PyString_FromString}{const char *v}
   Returns a new string object with the value \var{v} on success, and
-  \NULL{} on failure.
+  \NULL{} on failure.  The parameter \var{v} must not be \NULL; it
+  will not be checked.
 \end{cfuncdesc}
 
 \begin{cfuncdesc}{PyObject*}{PyString_FromStringAndSize}{const char *v,