Add a note for $*IN and friends to have a look at other documentation.

See issue #1955

doc/Language/variables.pod6

@@ -1226,6 +1226,10 @@ X<|$*HOME>X<|$*SPEC>X<|$*TMPDIR>X<|$*THREAD>X<|$*SCHEDULER>X<|$*USAGE>
     ------------------+--------------------------------------------
 =end table
 
+With particular regard to file handles C<$*IN>, C<$*OUT> and C<$*ERR> please see also
+the L<Input and Output|/language/io> page and the L<IO::Special> class.
+
+
 Note on usage of $*SCHEDULER:
 
 For the current Rakudo, by default this imposes a maximum of 16 threads on the methods