add method .multi-invocant (#4647)

Author: arkiuat

doc/Type/Parameter.rakudoc

@@ -258,6 +258,17 @@ L<invocant parameter|/language/signatures#Parameter_separators>.
     say $sig.params[0].invocant;                       # OUTPUT: «True␤»
     say $sig.params[1].invocant;                       # OUTPUT: «False␤»
 
+=head2 method multi-invocant
+
+    method multi-invocant(Parameter:D: --> Bool:D)
+
+Returns C<True> if the parameter affects 
+L<multi dispatch|/language/functions#Multi-dispatch>.
+
+    my Signature $sig = :($a;; $b);
+    say $sig.params[0].multi-invocant;                 # OUTPUT: «True␤»
+    say $sig.params[1].multi-invocant;                 # OUTPUT: «False␤»
+
 =head2 method default
 
     method default(Parameter:D: --> Code:_)