Document .pid and add some notes to .ready

Author: AlexDaniel

doc/Type/Proc/Async.pod6

@@ -287,6 +287,21 @@ Returns a L<Promise|/type/Promise> that will be kept once the process
 has successfully started. L<Promise|/type/Promise> will be broken if the program
 fails to start.
 
+B<Implementation-specific note:> Starting from Rakudo 2018.04, the
+returned promise will hold the process id (PID).
+
+=head2 method pid
+
+    method pid(Proc::Async:D: --> Promise:D)
+
+Equivalent to L<ready>.
+
+Returns a L<Promise|/type/Promise> that will be kept once the process
+has successfully started. L<Promise|/type/Promise> will be broken if the program
+fails to start.
+
+B<Implementation-specific note:> Available starting from Rakudo 2018.04.
+
 =head2 method path
 
     method path(Proc::Async:D:)