Update ldict-keys.

doc/utils.md

@@ -90,6 +90,12 @@ the dictionary does not contain the key ```k```, an error is raised.
 Sets existing key ```k``` to the new value ```v``` or inserts it if it
 is not already present in the dictionary ```ld```.
 
+    (ldict-keys ld)
+
+* ```ld``` - a ldict instance
+
+Returns the list of keys stored in given dictionary.
+
 ### IO
 
     (import util-io)