Expands explanation on is rw trait

With new examples, and a reference to `is rw` on the whole class. It
was probably better where it is, in classtut, rather than in the
typesystem, which didn't seem to have (at least now, 18 months after
OP) a place for it. This closes #1572

Author: JJ

doc/Language/classtut.pod6

@@ -257,16 +257,15 @@ without having to write the method by hand. You are free instead to write
 your own accessor method, if at some future point you need to do something
 more complex than return the value.
 
+X<|traits,is rw>
 Note that using the C<.> twigil has created a method that will provide
 read-only access to the attribute. If instead the users of this object
 should be able to reset a task's completion state (perhaps to perform it
 again), you can change the attribute declaration:
 
     has Bool $.done is rw;
 
-X<|traits,is rw>
-
-The C<is rw> trait causes the generated accessor method to return something
+The C<is rw> trait causes the generated accessor method to return a container
 external code can modify to change the value of the attribute.
 
 You can also supply default values to attributes (which works equally for
@@ -281,6 +280,19 @@ evaluated at that time, and can even reference earlier attributes:
 has Task @!dependencies;
 has $.ready = not @!dependencies;
 
+Writable attributes are accessible through writable containers:
+
+=for code
+class a-class {
+    has $.an-attribute is rw;
+}
+say (a-class.new.an-attribute = "hey"); # OUTPUT: «hey␤»
+
+This attribute can also be accessed using the C<.an-attribute> or
+C<.an-attribute()> syntax. See also
+L<the C<is rw> trait on classes|/language/typesystem#trait_is_rw> for examples
+on how this works on the whole class.
+
 X<|class variables>
 A class declaration can also include I<class variables>, which are variables
 that are shared by all instances, and can be used for things like counting the