additional javadoc for NfcAdapter.ReaderCallback

Change-Id: I3cad8b43d53992bb3d53920abb510456ee72afb5

core/java/android/nfc/NfcAdapter.java

@@ -49,6 +49,8 @@ import android.util.Log;
  * <h3>Developer Guides</h3>
  * <p>For more information about using NFC, read the
  * <a href="{@docRoot}guide/topics/nfc/index.html">Near Field Communication</a> developer guide.</p>
+ * <p>To perform basic file sharing between devices, read
+ * <a href="{@docRoot}training/beam-files/index.html">Sharing Files with NFC</a>.
  * </div>
  */
 public final class NfcAdapter {
@@ -309,8 +311,12 @@ public final class NfcAdapter {
     final Context mContext;
 
     /**
-     * A callback to be invoked when the system has found a tag in
-     * reader mode.
+     * A callback to be invoked when the system finds a tag while the foreground activity is
+     * operating in reader mode.
+     * <p>Register your {@code ReaderCallback} implementation with {@link
+     * NfcAdapter#enableReaderMode} and disable it with {@link
+     * NfcAdapter#disableReaderMode}.
+     * @see NfcAdapter#enableReaderMode
      */
     public interface ReaderCallback {
         public void onTagDiscovered(Tag tag);