Add a note that friends are not found by unqualified lookup.

Author: jensmaurer

source/classes.tex

@@ -6644,6 +6644,9 @@
 \begin{itemize}
 \item a non-static const member of \tcode{C} having one parameter of type \tcode{const C\&}, or
 \item a friend of \tcode{C} having two parameters of type \tcode{const C\&}.
+\begin{note}
+Such a friend is not visible to unqualified lookup\iref{namespace.memdef}.
+\end{note}
 \end{itemize}
 
 \pnum