[const.iterators.ops] Add missing \pnum and replace returns with effects (#6581)

Author: hewillk

source/iterators.tex

@@ -4670,10 +4670,11 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
 Let \tcode{\placeholder{op}} be the operator.
 
 \pnum
-\returns
+\effects
 Equivalent to: \tcode{return x \placeholder{op} y.\exposid{current_};}
 \end{itemdescr}