Document is required's value is a 6.d thing

Closes #2184 D#2184

doc/Type/Attribute.pod6

@@ -62,7 +62,9 @@ error.
     CATCH{ default { say .^name, ': ', .Str } }
     # OUTPUT: «X::Attribute::Required: The attribute '$!a' is required, but you did not provide a value for it.␤»
 
-You can also specify a reason why the attribute is required:
+B<Available as of 6.d language version> (early implementation exists in Rakudo compiler 2018.07+):
+
+You can specify a reason why the attribute is required:
 
     class D {
         has $.a is required("it is a good idea");