[dcl.fct] Add cross-reference pointing to [depr.ellipsis.comma]

Author: tkoeppe

source/declarations.tex

@@ -3688,7 +3688,7 @@
 A \grammarterm{parameter-declaration-clause}
 of the form
 \grammarterm{parameter-declaration-list} \tcode{...}
-is deprecated.
+is deprecated\iref{depr.ellipsis.comma}.
 \begin{example}
 The declaration
 \begin{codeblock}