In Attribute.rakudoc, create section for trait is readonly (#4692)

* create section for trait is readonly

* correct signature of trait readonly 

Make the signature match exactly the (only) one obtained via introspection, e.g. via: 

&trait_mod:<is>.candidates».signature».raku.grep(/readonly/).say;

* skip CI test for error-illustrating code

* skip CI test for signature listing

* remove erroneous $

compare output of introspection
`&trait_mod:<is>.candidates».signature».raku.grep(/ \(\$\) /)».say`

* remove erroneous `my $`

* remove `=for code`

* correct comma to period

Author: schultzdavid

doc/Type/Attribute.rakudoc

@@ -49,7 +49,7 @@ BEGIN {
 
 =head1 Traits
 
-=head2 X<Trait is default|Traits,is default>
+=head2 X<trait is default|Traits,is default>
 
 An attribute that is assigned L<C<Nil>|/type/Nil> will revert to its default value
 set with the trait C<is default>. In the case of arrays or associatives, the
@@ -70,7 +70,7 @@ argument of C<is default> will set the default item value or hash value.
     $foo.bar =Nil;
     say $foo; # OUTPUT: «Foo.new(bar => [42])␤»
 
-=head2 X<Trait is required|Traits,is required>
+=head2 X<trait is required|Traits,is required>
 
     multi trait_mod:<is> (Attribute $attr, :$required!)
 
@@ -151,6 +151,27 @@ default accessor for the attribute will return a writable value.
    CATCH { default { put .^name, ': ', .Str } };
    # OUTPUT: «X::Assignment::RO: Cannot modify an immutable Any␤»
 
+=head2 X<trait is readonly|Traits,is readonly>
+
+    multi trait_mod:<is> (Attribute:D $attr, :readonly($)!)
+
+Marks as C<readonly> an attribute of a class that has the C<is rw> trait.
+
+    =begin code :skip-test<illustrates error>
+    class Thing is rw {
+        has $.form;
+        has $.matter is readonly;
+    };
+
+    my $t = Thing.new(matter => "copper", form => "cubic");
+    $t.form = "round";   # OK, form is rw
+    $t.matter = "iron";  # not OK, matter is readonly
+    # OUTPUT: «Cannot modify an immutable Str (copper)␤...»
+    =end code
+
+The attributes of classes without the C<is rw> trait are already readonly by default.
+Thus, in classes without the C<is rw> trait, the attribute trait C<is readonly> is redundant.
+
 =head2 X<trait is built|Traits,is built>
 
     multi trait_mod:<is>(Attribute:D $a, :$built!)