Adds reason for is required, refs #2632

Author: JJ

doc/Language/typesystem.pod6

@@ -433,6 +433,7 @@ methods on all public attributes of that class.
 Defined as:
 
     multi sub trait_mod:<is>(Attribute $attr, :$required!)
+    multi sub trait_mod:<is>(Parameter:D $param, :$required!)
 
 Marks a class or roles attribute as required. If the attribute is not
 initialized at object construction time throws
@@ -452,6 +453,15 @@ L<X::Attribute::Required>.
     CATCH { default { say .^name => .Str } }
     # OUTPUT: «X::Attribute::Required => The attribute '$!attr' is required, but you did not provide a value for it.␤»
 
+You can provide a reason why it's required as an argument to C<is required>
+
+=for code :skip-test
+class Correct {
+    has $.attr is required("it's so cool")
+};
+say Correct.new();
+# OUTPUT: «The attribute '$!attr' is required because it's so cool,␤but you did not provide a value for it.␤»
+
 =head3 trait C<hides>
 
 The trait C<hides> provides inheritance without being subject to