Document TransitionLock semantics and RCU intent

Rumor has it, NT is one of the few kernels that actually has RCU
(alongside Linux and maybe some IBM things). So maybe if we ever learn
how to use NT's RCU functionality, we'll be able to replace this with
that.

wintun.c

@@ -56,6 +56,10 @@ typedef enum _TUN_STATE {
 typedef struct _TUN_CTX {
 	volatile TUN_STATE State;
 	volatile NDIS_DEVICE_POWER_STATE PowerState;
+
+	/* Used like RCU. When we're making use of queues, we take a shared lock. When we want to
+	 * drain the queues and toggle the state, we take an exclusive lock before toggling the
+	 * atomic and then releasing. It's similar to setting the atomic and then calling rcu_barrier(). */
 	EX_SPIN_LOCK TransitionLock;
 
 	NDIS_HANDLE MiniportAdapterHandle;