Updated the text about multi-dispatch.

Closes #858

Author: Jan-Olof Hendig

doc/Language/functions.pod6

@@ -212,8 +212,10 @@ You declare each candidate with the C<multi> declarator:
     congratulate 'Larry';       # OUTPUT: «Happy birthday, Larry␤»
     congratulate 'Bob', 45;     # OUTPUT: «Happy 45th birthday, Bob␤»
 
-Dispatch can happen on the number of arguments (the L<arity|/type/Routine#(Code)_method_arity>), but also on the
-type:
+Dispatch can happen on the number of arguments (the L<arity|/type/Routine#(Code)_method_arity>), the
+type of arguments but also on additional assertions which can be placed on them. For more information
+about type constraints see the documentation for the L<Signature|/type/Signature#Type_Constraints>
+class.
 
     multi as-json(Bool $d) { $d ?? 'true' !! 'false'; }
     multi as-json(Real $d) { ~$d }