Add some javadoc to Provider.java

Author: SUPERCILEX

auth/src/main/java/com/firebase/ui/auth/provider/Provider.java

@@ -4,13 +4,21 @@
 import android.content.Context;
 import android.content.Intent;
 
+import com.google.firebase.auth.GoogleAuthProvider;
+
 public interface Provider {
     /** Retrieves the name of the IDP, for display on-screen. */
     String getName(Context context);
 
+    /** Retrieves the id of the IDP, e.g. {@link GoogleAuthProvider#PROVIDER_ID}. */
     String getProviderId();
 
+    /** Start the login process for the IDP, e.g. show the Google sign-in activity. */
     void startLogin(Activity activity);
 
+    /**
+     * Handle the sign result by either finishing the calling activity or sending an {@link
+     * IdpProvider.IdpCallback} response.
+     */
     void onActivityResult(int requestCode, int resultCode, Intent data);
 }