Clarifies combinations, refs #2632

Author: JJ

doc/Type/Any.pod6

@@ -1327,6 +1327,13 @@ method and uses L«C<List.combinations>|/type/List#routine_combinations» on it.
 =for code
 say (^3).combinations; # OUTPUT: «(() (0) (1) (2) (0 1) (0 2) (1 2) (0 1 2))␤»
 
+Combinations on an empty data structure will return a list with a single
+element, an empty list; on a data structure with a single element it will
+return a list with two lists, one of them empty and the other with a single
+element.
+
+    say set().combinations; # OUTPUT: «(())␤»
+
 =head2 method grep
 
 Defined as: