[dcl.dcl] Improve note regarding nodeclspec-function-declarations

Author: sdkrystian

source/declarations.tex

@@ -216,8 +216,9 @@
 A \grammarterm{nodeclspec-function-declaration} shall declare a
 constructor, destructor, or conversion function.
 \begin{note}
-A \grammarterm{nodeclspec-function-declaration} can only be used in a
-\grammarterm{template-declaration}\iref{temp.pre},
+Because a member function cannot be subject to a non-defining declaration
+outside of a class definition\iref{class.mfct}, a \grammarterm{nodeclspec-function-declaration}
+can only be used in a \grammarterm{template-declaration}\iref{temp.pre},
 \grammarterm{explicit-instantiation}\iref{temp.explicit}, or
 \grammarterm{explicit-specialization}\iref{temp.expl.spec}.
 \end{note}