clarify what arguments are defined in what where-clause

Author: gfldex

doc/Type/Signature.pod6

@@ -138,6 +138,17 @@ The first of those can be shortened to
 i.e., you can use a literal directly as a type and value constraint
 on an anonymous parameter.
 
+Any argument in a C<Signature> is declared in a C<where>-clause that is
+following that argument. Therefor, the C<where>-clause of the last argument got
+access to all arguments of a signature that are not part of a sub-signature.
+For sub-signature place the C<where>-clause inside the sub-signature.
+
+    sub one-of-them(:$a, :$b, :$c where { $a.defined ^^ $b.defined ^^ $c.defined } ) {
+        $a // $b // $c
+    };
+    say one-of-them(c=>42);
+    # OUTPUT«42␤»
+
 =head3 Constraining Slurpy Arguments
 
 L<Slurpy arguments|Slurpy_(A.K.A._Variadic)_Parameters> can not have type