[ios, macos] Clarify API docs for point collection

Author: boundsj

platform/darwin/src/MGLPointCollection.h

@@ -5,8 +5,13 @@
 #import "MGLShape.h"
 
 /**
- The `MGLPointCollection` class is used to define shapes composed of multiple 
- points.
+ The `MGLPointCollection` class is used to define an array of coordinates.
+ 
+ @note `MGLPointCollection` objects cannot be added to a map view using
+ `-[MGLMapView addAnnotations:]` and related methods. However, when used in a
+ `MGLPointCollectionFeature` to initialize a `MGLGeoJSONSource` that is added
+ to the map view's style, the point collection represents as a group of distinct 
+ annotations.
  */
 @interface MGLPointCollection : MGLShape <MGLOverlay>
 

platform/ios/src/MGLMapView.h

@@ -895,11 +895,10 @@ IB_DESIGNABLE
 /**
  Adds an annotation to the map view.
  
- @note `MGLMultiPolyline`, `MGLMultiPolygon`, and `MGLShapeCollection` objects
-    cannot be added to the map view at this time. Nor can `MGLMultiPoint`
-    objects that are not instances of `MGLPolyline` or `MGLPolygon`. Any
-    multipoint, multipolyline, multipolygon, or shape collection object that is
-    specified is silently ignored.
+ @note `MGLMultiPolyline`, `MGLMultiPolygon`, `MGLShapeCollection`, and 
+    `MGLPointCollection` objects cannot be added to the map view at this time. 
+    Any multipoint, multipolyline, multipolygon, shape collection or point 
+    collection, object that is specified is silently ignored.
  
  @param annotation The annotation object to add to the receiver. This object
     must conform to the `MGLAnnotation` protocol. The map view retains the