Rename RAKUDO_EXCEPTIONS_HANDLER

Procspec: Raku/roast@7d2432590e
Rakudo fix: rakudo/rakudo@5e1df41b34

doc/Programs/03-environment-variables.pod6

@@ -22,10 +22,12 @@ information to standard error.
 
 =head2 Error Message Verbosity and Strictness
 
-=item C<RAKUDO_EXCEPTIONS_HANDLER> If present, the print_exception routine will
+=item C<PERL6_EXCEPTIONS_HANDLER> If present, the print_exception routine will
 use a class of that name to process the exception for output. Rakudo currently
 ships with Exceptions::JSON (invoked by setting this variable to C<JSON>, to
-override the default output.
+override the default output. B<NOTE:> this env var was added in 6.d language.
+Early implementation is available in Rakudo compiler as of version 2018.08,
+and before that it was available as C<RAKUDO_EXCEPTIONS_HANDLER>.
 
 =item C<RAKUDO_NO_DEPRECATIONS>