crtools: add comments to fields of struct pid

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/crtools.h

@@ -178,8 +178,8 @@ struct rst_info {
 
 struct pid
 {
-	u32 real_pid;
-	u32 pid;
+	u32 real_pid;		/* used to peek/poke tasks during dump stage */
+	u32 pid;		/* used all over in the images and saved after restore */
 };
 
 struct pstree_item {