Adds description of .raku for Mu

With its two differences closes #3304

Author: JJ

doc/Type/Mu.pod6

@@ -180,6 +180,21 @@ ways to write a Raku expression that produces a particular value. Since the
 change of name to Raku, this method is deprecated and might disappear in the
 near future. Use C<.raku> instead.
 
+=head2 method raku
+
+    multi method raku(Mu:U:)
+    multi method raku(Mu:D:)
+
+For type objects, returns its name if C<.perl> has not been redefined from
+Mu, or calls C<.perl> o the name of the type object otherwise.
+
+    say Str.raku;          # OUTPUT: «Str␤»
+
+For plain objects, it will conventionally return a representation of the object
+that can be used via EVAL to reconstruct the value of the object.
+
+    say (1..3).Set.raku;  # OUTPUT: «Set.new(1,2,3)␤»
+
 =head2 method item
 
     method item(Mu \item:) is raw