fix list comprehension discussion to use \keyword macro instead of simply

quoting keywords.

Doc/ref/ref5.tex

@@ -164,10 +164,11 @@ by providing either a list of expressions or a list comprehension.
 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
-single expression followed by at least one "for" clause and zero or  more
-"for" or "if" clauses.  In this 
+single expression followed by at least one \keyword{for} clause and zero or
+more \keyword{for} or \keyword{if} clauses.  In this 
 case, the elements of the new list are those that would be produced
-by considering each of the "for" or "if" clauses a block, nesting from
+by considering each of the \keyword{for} or \keyword{if} clauses a block,
+nesting from 
 left to right, and evaluating the expression to produce a list element
 each time the innermost block is reached.
 \obindex{list}