[ios] Improve user location annotation documentation

- Add note to mapView:viewForAnnotation: delegate method.
- Edit docs in iosapp's implementation of that delegate method.

platform/ios/app/MBXViewController.m

@@ -732,15 +732,17 @@ static NSString * const MBXViewControllerAnnotationViewReuseIdentifer = @"MBXVie
         annotationView = [[MBXAnnotationView alloc] initWithReuseIdentifier:MBXViewControllerAnnotationViewReuseIdentifer];
         annotationView.frame = CGRectMake(0, 0, 10, 10);
         annotationView.backgroundColor = [UIColor whiteColor];
-        
-        // uncomment to make the annotation view draggable
-        // also note that having two long press gesture recognizers on overlapping views (`self.view` & `annotationView`) will cause weird behaviour
-        // comment out the pin dropping functionality in the handleLongPress: method in this class to make draggable annotation views play nice
+
+        // Note that having two long press gesture recognizers on overlapping
+        // views (`self.view` & `annotationView`) will cause weird behaviour.
+        // Comment out the pin dropping functionality in the handleLongPress:
+        // method in this class to make draggable annotation views play nice.
         annotationView.draggable = YES;
         
-        // uncomment to force annotation view to maintain a constant size when the map is tilted
-        // by default, annotation views will shrink and grow as the move towards and away from the
-        // horizon. Relatedly, annotations backed by GL sprites ONLY scale with viewing distance currently.
+        // Uncomment to force annotation view to maintain a constant size when
+        // the map is tilted. By default, annotation views will shrink and grow
+        // as they move towards and away from the horizon. Relatedly, annotations
+        // backed by GL sprites currently ONLY scale with viewing distance.
         // annotationView.scalesWithViewingDistance = NO;
     } else {
         // orange indicates that the annotation view was reused

platform/ios/src/MGLMapViewDelegate.h

@@ -273,6 +273,11 @@ NS_ASSUME_NONNULL_BEGIN
  Touch frameworks. On the other hand, static annotation images use less memory
  and draw more quickly than annotation views.
  
+ The user location annotation view can also be customized via this method. When
+ `annotation` is an instance of `MGLUserLocation` (or equal to the map view’s
+ `userLocation` property), return an instance of `MGLUserLocationAnnotationView`
+ (or a subclass thereof).
+
  @param mapView The map view that requested the annotation view.
  @param annotation The object representing the annotation that is about to be
     displayed.