Document thread-safety requirements for ARTDeviceStorage

This doesn’t change anything, it just explains the current behaviour of
the code.

ARTLocalDeviceStorage already conforms to these requirements, since it
uses the (already thread-safe) user defaults / keychain. However,
MockDeviceStorage does need some synchronisation. I’ll do that next.

Source/ARTDeviceStorage.h

@@ -3,6 +3,8 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+// Instances of ARTDeviceStorage should expect to have their methods called
+// from any thread.
 @protocol ARTDeviceStorage <NSObject>
 - (nullable id)objectForKey:(NSString *)key;
 - (void)setObject:(nullable id)value forKey:(NSString *)key;