Update for existsence of P5getpriority

- add description for getppid
- mention P5getpriority for getpriority / setpriority / getpgrp

Author: lizmat

doc/Language/5to6-perlfunc.pod6

@@ -576,13 +576,28 @@ implemented.
 
 =item getpgrp PID
 
-Does not appear to be implemented.
+Will not be implemented.
+
+The Perl 6 ecosystem has a module C<P5getpriority> which exports a C<getpgrp>
+function that mimics the original Perl 5 behaviour as much as possible.
+
+=head2 getppid
+
+=item getppid PID
+
+Will not be implemented.
+
+The Perl 6 ecosystem has a module C<P5getpriority> which exports a C<getppid>
+function that mimics the original Perl 5 behaviour as much as possible.
 
 =head2 getpriority
 
 =item getpriority WHICH, WHO
 
-Does not appear to be implemented.
+Will not be implemented.
+
+The Perl 6 ecosystem has a module C<P5getpriority> which exports a C<getpriority>
+function that mimics the original Perl 5 behaviour as much as possible.
 
 =head2 get and set functions
 
@@ -1533,7 +1548,10 @@ No longer in core. Will probably wind up in a POSIX module.
 
 =item setpriority WHICH, WHO, PRIORITY
 
-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<setpriority>
+function that mimics the original Perl 5 behaviour as much as possible.
 
 =head2 setsockopt