update doc comments

Author: morganchen12

FirebaseDatabaseUI/FirebaseIndexArray.h

@@ -98,13 +98,19 @@ didFailLoadWithError:(NSError *)error;
 
 /**
  * Delegate method which is called whenever the backing query is canceled. This error is fatal
- * and the index array will become unusable afterward, so please handle it appropriately.
+ * and the index array will become unusable afterward, so please handle it appropriately
+ * (i.e. by displaying a modal error explaining why there's no content).
  * @param error the error that was raised
  */
 - (void)array:(FirebaseIndexArray *)array queryCancelledWithError:(NSError *)error;
 
 @end
 
+/**
+ * A FirebaseIndexArray instance uses a query's contents to query children of
+ * a separate database reference, which is useful for displaying an indexed list
+ * of data as described in https://firebase.google.com/docs/database/ios/structure-data
+ */
 @interface FirebaseIndexArray : NSObject
 
 /**

FirebaseDatabaseUI/FirebaseIndexCollectionViewDataSource.h

@@ -50,7 +50,11 @@ didFailLoadAtIndex:(NSUInteger)index
 
 @end
 
-
+/**
+ * An object that manages a @c FirebaseIndexArray and uses it to populate and update
+ * a collection view with a single section. The data source maintains a reference to but
+ * does not claim ownership of the collection view that it updates.
+ */
 @interface FirebaseIndexCollectionViewDataSource : NSObject <UICollectionViewDataSource>
 
 @property (nonatomic, readwrite, weak, nullable) id<FirebaseIndexCollectionViewDataSourceDelegate> delegate;

FirebaseDatabaseUI/FirebaseIndexTableViewDataSource.h

@@ -50,6 +50,11 @@ didFailLoadAtIndex:(NSUInteger)index
 
 @end
 
+/**
+ * An object that manages a @c FirebaseIndexArray and uses it to populate and update
+ * a table view with a single section. The data source maintains a reference to but
+ * does not claim ownership of the table view that it updates.
+ */
 @interface FirebaseIndexTableViewDataSource : NSObject <UITableViewDataSource>
 
 /**