Improve explanation of reference counts for PyString_Concat().

Doc/api/api.tex

@@ -1791,8 +1791,9 @@ Resturns a \NULL{} terminated representation of the contents of \var{string}.
 \begin{cfuncdesc}{void}{PyString_Concat}{PyObject **string,
                                          PyObject *newpart}
 Creates a new string object in \var{*string} containing the
-contents of \var{newpart} appended to \var{string}.  The old value of
-\var{string} have its reference count decremented.  If the new string
+contents of \var{newpart} appended to \var{string}; the caller will
+own the new reference.  The reference to the old value of \var{string}
+will be stolen.  If the new string
 cannot be created, the old reference to \var{string} will still be
 discarded and the value of \var{*string} will be set to
 \NULL{}; the appropriate exception will be set.