Update Parameter.sub_signature and add example

Author: stoned

doc/Type/Parameter.pod6

@@ -311,9 +311,21 @@ d("four", "five");  # Fails when binding $x
 
 =head2 method sub_signature
 
+Defined as:
+
+    method sub_signature(Parameter:D: --> Signature:_)
+
 If the parameter has a
 L<sub-signature|/type/Signature#Sub-signatures>,
-returns a C<Signature> object for it.  Otherwise returns C<Any>.
+returns a C<Signature> object for it.  Otherwise returns C<Signature>.
+
+B<Note:> Before Rakudo version 2020.08 the return value for a parameter
+with no sub-signature was C<Any>.
+
+=for code
+my Signature $sig = :(@array ($first, *@rest), @other);
+say $sig.params[0].sub_signature;     # OUTPUT:«($first, *@rest)␤»
+say $sig.params[1].sub_signature;     # OUTPUT:«(Signature)␤»
 
 =head2 method prefix