Update javadoc to give app write better guidence

Update javadoc of close methods of GATT cliet and server
Update javadoc of BluetoothAdapter
bug 8963528

Change-Id: I45ec618fd495225ed11a6171f33bfdc218397d4c
diff --git a/core/java/android/bluetooth/BluetoothGatt.java b/core/java/android/bluetooth/BluetoothGatt.java
index bffe64b..69dc4e2 100644
--- a/core/java/android/bluetooth/BluetoothGatt.java
+++ b/core/java/android/bluetooth/BluetoothGatt.java
@@ -565,6 +565,9 @@
 
     /**
      * Close this Bluetooth GATT client.
+     *
+     * Application should call this method as early as possible after it is done with
+     * this GATT client.
      */
     public void close() {
         if (DBG) Log.d(TAG, "close()");