[dcl.typedef] Split paragraph on typedef name for linkage into two parts

(how you know when you have one, and the restrictions on types that have
one).

Author: zygoloid

source/declarations.tex

@@ -738,6 +738,8 @@
 typedef decltype([]{}) C;       // the closure type has no name for linkage purposes
 \end{codeblock}
 \end{example}
+
+\pnum
 An unnamed class with a typedef name for linkage purposes shall not
 \begin{itemize}
 \item