Add missing sentence

Otherwise it's unclear why .pid is called like that.

Author: AlexDaniel

doc/Type/Proc/Async.pod6

@@ -298,7 +298,7 @@ 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.
+fails to start. Returned promise will hold the process id (PID).
 
 B<Implementation-specific note:> Available starting from Rakudo 2018.04.