Improves the documentation of .serial

According to @lizmat comments in
https://stackoverflow.com/questions/56057931/what-does-serial-do and
@jnthn in the #1803 issue. I've finally opted for leaving it there, on
the premises that it might benefit no one if it disappears. I've added
all the necessary caveats. This closes #1803

doc/Type/Any.pod6

@@ -394,15 +394,24 @@ Defined as
 
     multi method serial()
 
-Returns the self-reference to the instance itself:
+This method is Rakudo specific, and is not included in the Perl 6 spec.
+
+The method returns the self-reference to the instance itself:
 
 =begin code
-my $b; # defaults to Any
+my $b;                 # defaults to Any
 say $b.serial.^name;   # OUTPUT: «Any␤»
+say $b.^name;          # OUTPUT: «Any␤»
 my $breakfast = 'food';
 $breakfast.serial.say; # OUTPUT: «food␤»
 =end code
 
+This is apparently a no-op, as exemplified by the third example above. However,
+in L<C<HyperSeq>|/type/HyperSeq>s and L<C<RaceSeq>|/type/RaceSeq>s it returns a
+serialized C<Seq>, so it can be considered the opposite of the C<hyper/race>
+methods. As such, it ensures that we are in serial list-processing mode, as
+opposed to the autothreading mode of those methods.
+
 =head2 method Hash
 
 Defined as: