add index entries for list comprehensions

Doc/ref/ref5.tex

@@ -147,6 +147,7 @@ pass uncaught.
 
 \subsection{List displays\label{lists}}
 \indexii{list}{display}
+\indexii{list}{comprehensions}
 
 A list display is a possibly empty series of expressions enclosed in
 square brackets:
@@ -161,6 +162,7 @@ list_if:     "if" test [list_iter]
 
 A list display yields a new list object.  Its contents are specified
 by providing either a list of expressions or a list comprehension.
+\indexii{list}{comprehensions}
 When a comma-separated list of expressions is supplied, its elements are
 evaluated from left to right and placed into the list object in that
 order.  When a list comprehension is supplied, it consists of a