docs: changes to broadcast documentation

- move BroadcastReceiver info to developer guide. see cl/140402421
- add usage note to CONNECTIVITY_ACTION broadcast

bug:32533262
bug:33106411

Change-Id: Ic2aa517831d29418e0c42aa6fc1e7f9aeb50f802

core/java/android/net/ConnectivityManager.java

@@ -87,6 +87,13 @@ public class ConnectivityManager {
      * sent as an extra; it should be consulted to see what kind of
      * connectivity event occurred.
      * <p/>
+     * Apps targeting Android 7.0 (API level 24) and higher do not receive this
+     * broadcast if they declare the broadcast receiver in their manifest. Apps
+     * will still receive broadcasts if they register their
+     * {@link android.content.BroadcastReceiver} with
+     * {@link android.content.Context#registerReceiver Context.registerReceiver()}
+     * and that context is still valid.
+     * <p/>
      * If this is a connection that was the result of failing over from a
      * disconnected network, then the FAILOVER_CONNECTION boolean extra is
      * set to true.