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

Author: jensmaurer

source/classes.tex

@@ -6664,6 +6664,9 @@
 \begin{codeblock}
 friend bool operator==(const X&, const X&);
 \end{codeblock}
+\begin{note}
+Such a friend function is not visible to unqualified lookup\iref{namespace.memdef}.
+\end{note}
 The operator is a \tcode{constexpr} function if its definition
 would satisfy the requirements for a \tcode{constexpr} function.
 \begin{note}