[ios, macos] Note that MGLShapeSource.shape must be set on main thread (#7687)

* Add Clarification for modifying ShapeCollections

Spinoff task from https://github.com/mapbox/mapbox-gl-native/issues/7622

* update wording for both get/set, remove whitespace

platform/darwin/src/MGLShapeSource.h

@@ -192,6 +192,9 @@ extern const MGLShapeSourceOption MGLShapeSourceOptionSimplificationTolerance;
  If the receiver was initialized using `-initWithIdentifier:URL:options:`, this
  property is set to `nil`. This property is unavailable until the receiver is
  passed into `-[MGLStyle addSource:]`.
+ 
+ You can get/set the shapes within a collection via this property. Actions must 
+ be performed on the application's main thread.
  */
 @property (nonatomic, copy, nullable) MGLShape *shape;