Rewrite rakudo-specific note

Normally implementation-specific things are not documented, but in
this particular case we can prevent a lot of banging against a wall by
mentioning some known issues. In this case the behavior has changed
quite a bit so IMO it's a worthy mention.

See issue #1486 for more info on docs vs implementation-specific notes.

doc/Type/Supply.pod6

@@ -777,11 +777,14 @@ The list of supported signals can be found by checking C<Signal::.keys> (as
 you would any enum). For more details on how enums work see
 L<enum|/language/typesystem#enum>.
 
-B<Note:> Currently L<Rakudo|/language/glossary#Rakudo> has a bug which
-makes it use numeric values of signals that may be incorrect for a
-particular system. For example, C<Signal(10)> may return C<SIGBUS>
-even if it is actually C<SIGUSR1> on your system.
-L<RT #132012|https://rt.perl.org/Ticket/Display.html?id=132012>
+B<Note:> L<Rakudo|/language/glossary#Rakudo> versions up to 2018.05
+had a bug due to which numeric values of signals were incorrect on
+some systems. For example, C<Signal(10)> was returning C<SIGBUS> even
+if it was actually C<SIGUSR1> on a particular system. That being said,
+using C<signal(SIGUSR1)> was working as expected on all Rakudo
+versions except 2018.04, 2018.04.1 and 2018.05, where the intended
+behavior can be achieved by using C<signal(SIGBUS)> instead. These
+issues are resolved in Rakudo releases after 2018.05.
 
 =head2 method IO::Notification.watch-path