Reimplement and document ldict-filter.

doc/utils.md

@@ -112,6 +112,15 @@ if it accepts three arguments, the key, value and numeric index
 In all cases the value the procedure returns is taken as the new value
 for given key.
 
+    (ldict-filter pred? ld)
+
+* ```pred?``` - predicate procedure
+* ```ld``` -a ldict instance
+
+Returns a new dictionary containing only key/value pairs matching the
+given predicate. The procedure ```pred?``` must accept two arguments -
+the key and the value.
+
 ### IO
 
     (import util-io)