[expr.prim.lambda.closure] Insert an extra \pnum (#6594)

Author: AlisdairM

source/expressions.tex

@@ -1872,6 +1872,8 @@
 types of a \grammarterm{lambda-declarator} do not affect these associated namespaces and
 classes.
 \end{note}
+
+\pnum
 The closure type is not an aggregate type\iref{dcl.init.aggr} and
 not a structural type\iref{temp.param}.
 An implementation may define the closure type differently from what