nvme: Add a clarifying comment

While it is easy enough to bounce over to nvme.c from nvme_ctrlr.c to
find this out, I've had to do that several times, so a little bit of
context is quite helpful.

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sys/dev/nvme/nvme_ctrlr.c

@@ -1588,6 +1588,12 @@ nvme_ctrlr_construct(struct nvme_controller *ctrlr, device_t dev)
 	return (0);
 }
 
+/*
+ * Called on detach, or on error on attach. The nvme_controller won't be used
+ * again once we return, so we have to tear everything down (so nothing
+ * references this, no callbacks, etc), but don't need to reset all the state
+ * since nvme_controller will be freed soon.
+ */
 void
 nvme_ctrlr_destruct(struct nvme_controller *ctrlr, device_t dev)
 {