[dcl.fct] Add deprecation notice and reword sentence

Author: eisenwave

source/declarations.tex

@@ -3661,9 +3661,13 @@
 argument and are not function parameter packs.
 Where syntactically correct and where ``\tcode{...}'' is not
 part of an \grammarterm{abstract-declarator},
-``\tcode{, ...}''
+``\tcode{...}''
 is synonymous with
-``\tcode{...}''.
+``\tcode{, ...}''.
+A \grammarterm{parameter-declaration-clause}
+of the form
+\grammarterm{parameter-declaration-list} \tcode{...}
+is deprecated.
 \begin{example}
 The declaration
 \begin{codeblock}