Adds indirect invocation to syntax

And also adds preamble to example, instead of skipping test. Closes #2131

Author: JJ

doc/Language/syntax.pod6

@@ -733,11 +733,18 @@ are subroutines invoked against an object (i.e., a class instance).
 Within a method the special variable C<self> contains the object instance
 (see L<Methods|/language/classtut#Methods>).
 
-=begin code :skip-test
+=begin code :preamble<class Person {
+    has Int $.age;
+    has Str $.name;
+    method set-name-age( $!name, $!age ) {}
+}
+my Person $person .= new;
+>
 # Method invocation. Object (instance) is $person, method is set-name-age
 $person.set-name-age('jane', 98);   # Most common way
 $person.set-name-age: 'jane', 98;   # Precedence drop
 set-name-age($person: 'jane', 98);  # Invocant marker
+set-name-age $person: 'jane', 98;   # Indirect invocation
 =end code
 
 For more information see L<functions|/language/functions>.