Better comment doc for expected value cache property

Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>

src/darwin/Framework/CHIP/MTRDevice.mm

@@ -187,7 +187,7 @@ @interface MTRDevice ()
 @property (nonatomic) NSMutableDictionary<MTRAttributePath *, NSDictionary *> * readCache;
 
 // Expected value cache is attributePath => NSArray of [NSDate of expiration time, NSDictionary of value, expected value ID]
-// - MTRDeviceExpectedValueFieldIndex
+// - See MTRDeviceExpectedValueFieldIndex for the definitions of indices into this array.
 // See MTRDeviceResponseHandler definition for value dictionary details.
 @property (nonatomic) NSMutableDictionary<MTRAttributePath *, NSArray *> * expectedValueCache;
 @property (nonatomic) uint64_t expectedValueNextID;