sets: update docs for "combine".

core/sets/sets-docs.factor

@@ -192,5 +192,5 @@ HELP: cardinality
 { $description "Returns the number of elements in the set. All sets support this operation." } ;
 
 HELP: combine
-{ $values { "sets" "a sequence of sets" } { "set" set } }
+{ $values { "sets" "a sequence of sets" } { "set/f" "a " { $link set } " or " { $link f } } }
 { $description "Outputs the union of a sequence of sets, or " { $link f } " if the sequence is empty." } ;

core/sets/sets.factor

@@ -116,7 +116,7 @@ M: sequence null?
 M: sequence cardinality
     pruned length ;
 
-: combine ( sets -- set )
+: combine ( sets -- set/f )
     [ f ]
     [ [ [ members ] map concat ] [ first ] bi set-like ]
     if-empty ;