Adds reference to P6Opaque, refs #1863, #1957

Author: JJ

doc/Language/traits.pod6

@@ -34,7 +34,15 @@ also native.
 =item X<CUnion> is going to use the same representation as an C<union> in C; see
 L<this|/language/nativecall#CUnions> for an example.
 
+On the other hand, X<P6opaque> is the default representation used for all
+objects in Perl 6.
 
+    m: class Thar {};
+    say Thar.REPR;    #OUTPUT: «P6opaque␤»
+
+The L<meta object protocol|/language/mop> uses it by default for every object
+and class unless specified otherwise; for that reason, it is in general not
+necessary unless you are effectively working with that interface.
 
 =end pod