Clarifies say in Mu, closes #2619

doc/Type/Mu.pod6

@@ -425,20 +425,27 @@ stringifying non-C<Str> object using the C<.Str> method.
 
 =head2 method say
 
-    multi method say(--> Bool:D)
+    multi method say()
 
 Will L<C<say>|/type/IO::Handle#method_say> to
 L<standard output|/language/variables#index-entry-$*OUT>.
-To produce machine readable output use C<.put> instead.
 
     say 42;                 # OUTPUT: «42␤»
 
+What C<say> actually does is, thus, deferred to the actual subclass. In <most
+cases|/routine/say> it calls L<C<.gist>|/routine/gist> on the object, returning
+a compact string representation.
+
 In non-sink context, C<say> will always return C<True>.
 
-    say (1,[1,2],"foo",Mu).map: so *.say ;# OUTPUT: «1␤[1 2]␤foo␤(Mu)␤(True True True True)␤»
+    say (1,[1,2],"foo",Mu).map: so *.say ;
+    # OUTPUT: «1␤[1 2]␤foo␤(Mu)␤(True True True True)␤»
+
+However, this behavior is just conventional and you shouldn't trust on it for
+your code. It's useful, however, to explain certain behaviors.
 
-C<say> is first printing out, but the outermost C<say> is printing the C<True>
-values returned by the C<so> operation.
+C<say> is first printing out in C<*.say>, but the outermost C<say> is printing
+the C<True> values returned by the C<so> operation.
 
 =head2 method ACCEPTS