Update and extend Mail.ru OAuth2 docs (#73)

* Update and extend Mail.ru OAuth2 docs
* Fix header underline

Co-authored-by: Michal Čihař <michal@cihar.com>

docs/backends/mailru.rst

@@ -1,7 +1,23 @@
 Mail.ru OAuth
 =============
 
-Mail.ru uses OAuth2 workflow, to use it fill in settings::
+Mail.ru uses OAuth2 workflow. `Register new application`_ to use it and fill in settings::
 
+    SOCIAL_AUTH_MAILRU_KEY = ''
+    SOCIAL_AUTH_MAILRU_SECRET = ''
+
+
+Add ``social_core.backends.mailru.MRGOAuth2`` to ``AUTHENTICATION_BACKENDS`` to activate Mail.ru authorization.
+
+Legacy OAuth2 authorization
+---------------------------
+
+Also available ``social_core.backends.mailru.MailruOAuth2`` for authorization with ``connect.mail.ru`` server.
+
+`Create an app`_ and set following settings::
+    
     SOCIAL_AUTH_MAILRU_OAUTH2_KEY = ''
     SOCIAL_AUTH_MAILRU_OAUTH2_SECRET = ''
+
+.. _Register new application: https://oauth.mail.ru/app/
+.. _Create an app: https://api.mail.ru/sites/my/add