windows: document new ProcessId field

Change-Id: I761f962f30eb48471a0f033cf30ea6c428d3ce49
Reviewed-on: https://go-review.googlesource.com/c/sys/+/174237
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/svc/service.go

@@ -72,7 +72,7 @@ type Status struct {
 	Accepts    Accepted
 	CheckPoint uint32 // used to report progress during a lengthy operation
 	WaitHint   uint32 // estimated time required for a pending operation, in milliseconds
-	ProcessId  uint32
+	ProcessId  uint32 // if the service is running, the process identifier of it, and otherwise zero
 }
 
 // ChangeRequest is sent to the service Handler to request service status change.