Improve Callable signature unpack docs

- Get rid of all "reference"s
- Block and Sub doesn't cover all routines; use Callable type instead
- s/argument/parameter/
- Explain whitespace isn't allowed
- Indicate the shorthand only works with `&` sigils and show the long version
    that can be used with other sigils

doc/Type/Signature.pod6

@@ -276,16 +276,24 @@ The L<Classes and Objects|/language/classtut#Starting_with_class>
 document further elaborates on the concepts of instances and type
 objects and discovering them with the C<.DEFINITE> method.
 
-=head3 X«Constraining signatures of Callables|function reference (constrain)»
+=head3 X«Constraining signatures of Callables|Callable (constrain)»
 
-To constrain L<block|/type/Block> and L<subroutine|/type/Sub> references based
-on their signature write the signature after the argument name.
+A L<Callable> parameter can be constrained by its signature, by specifying
+a L<Signature> literal right after the parameter (no whitespace allowed):
 
     sub f(&c:(Int, Str))  { say c(10, 'ten') };
     sub g(Int $i, Str $s) { $s ~ $i };
     f(&g);
     # OUTPUT: «ten10␤»
 
+This shorthand syntax is available only for parameters with the `&` sigil. For
+others, you need to use the long version:
+
+    sub f($c: where .signature ~~ :(Int, Str))  { say $c(10, 'ten') };
+    sub g(Num $i, Str $s) { $s ~ $i };
+    f(&g);
+    # OUTPUT: «ten10␤»
+
 =head3 X«Constraining Return Types|-->;returns»
 
 There are multiple ways to constrain return types on a sub or method. All versions below