Update setpgrp documentation

Author: lizmat

doc/Language/5to6-perlfunc.pod6

@@ -1542,7 +1542,10 @@ Can be found in the C<IO::Socket> class.
 
 =item setpgrp PID, PGRP
 
-No longer in core. Will probably wind up in a POSIX module.
+Will not be implemented.
+
+The Perl 6 ecosystem has a module C<P5getpriority> which exports a C<setpgrp>
+function that mimics the original Perl 5 behaviour as much as possible.
 
 =head2 setpriority