Hint at the intended usage for the "ll" field of struct uuid_private.

Discussed with:	kib
MFC after:	1 week

sys/kern/kern_uuid.c

@@ -58,7 +58,7 @@ CTASSERT(sizeof(struct uuid) == 16);
 /* We use an alternative, more convenient representation in the generator. */
 struct uuid_private {
 	union {
-		uint64_t	ll;		/* internal. */
+		uint64_t	ll;	/* internal, for uuid_last only */
 		struct {
 			uint32_t	low;
 			uint16_t	mid;