Minimally document Proc.pid

Author: lizmat

doc/Type/Proc.pod6

@@ -144,6 +144,12 @@ through to the shell for parsing and execution.
 The command method is an accessor to an array containing the arguments that were
 passed when the Proc object was executed via C<spawn> or C<shell> or C<run>.
 
+=head2 method pid
+
+    method pid(Proc:D:)
+
+Returns the C<$*PID> value of the process if available, or C<Nil>.
+
 =head2 method exitcode
 
     method exitcode(Proc:D: --> Int:D)