RUBY-1734 Update accepted auth mechanisms in documentation (#1269)

Author: p-mongo

docs/tutorials/ruby-driver-authentication.txt

@@ -141,12 +141,35 @@ MongoDB, see the :manual:`SASL/LDAP tutorial in the MongoDB Manual
                              ssl_cert: '/path/to/client.pem',
                              ssl_ca_cert: '/path/to/ca.pem' )
 
+.. _kerberos:
+
 Kerberos (GSSAPI) mechanism
 ```````````````````````````
 *Requires MongoDB Enterprise Edition v2.4 or greater.*
 
 MongoDB Enterprise Edition v2.4+ supports Kerberos authentication.
 
+To use Kerberos authentication from Ruby,
+`mongo_kerberos <https://rubygems.org/gems/mongo_kerberos>`_ must be
+installed. Add to your ``Gemfile``:
+
+.. code-block:: plain
+
+  gem 'mongo'
+  gem 'mongo_kerberos'
+
+... and in your application:
+
+.. code-block:: ruby
+
+  require 'mongo'
+  require 'mongo_kerberos'
+
+To use Kerberos in the Ruby driver with **MRI**, create a
+ticket-granting ticket using ``kinit``. See 
+`this documentation <http://linux.die.net/man/1/kinit>`_ for more
+information.
+
 To use Kerberos in the Ruby driver with **JRuby**, do the following:
 
 1. Specify several system properties so that the underlying GSSAPI Java
@@ -158,12 +181,6 @@ To use Kerberos in the Ruby driver with **JRuby**, do the following:
 2. Either provide a password OR set the 'java.security.auth.login.config'
    system property to a config file that references a keytab file.
 
-To use Kerberos in the Ruby driver
-with **Matz's Ruby Interpreter (MRI)**, create a
-ticket-granting ticket using ``kinit``. See 
-`this documentation <http://linux.die.net/man/1/kinit>`_ for more
-information.
-
 For more information about deploying MongoDB with Kerberos
 authentication, see the :manual:`manual
 </tutorial/control-access-to-mongodb-with-kerberos-authentication/>`.

docs/tutorials/ruby-driver-create-client.txt

@@ -248,11 +248,17 @@ Ruby Options
 
    * - ``:auth_mech``
      - Specifies the authenticaion mechanism to use. Can be one of:
-       ``:mongodb_cr``, ``:mongodb_x509``, ``:plain``, ``:scram``.
+       ``:gssapi``, ``:mongodb_cr``, ``:mongodb_x509``, ``:plain``,
+       ``:scram``, ``:scram256``. GSSAPI (Kerberos) authentication
+       :ref:`requires additional dependencies <kerberos>`.
      - ``Symbol``
-     - MongoDB 3.0 and later: ``:scram`` if user credentials
-       are supplied but an ``:auth_mech`` is not. 2.6 and earlier:
-       ``:mongodb_cr``
+     - If user credentials are not supplied, ``nil``. If user credentials
+       are supplied, the default depends on server version.
+       MongoDB 4.0 and later: ``:scram256`` if user credentials correspond
+       to a user which supports SCRAM-SHA-256 authentication, otherwise
+       ``:scram``.
+       MongoDB 3.0-3.6: ``:scram``.
+       MongoDB 2.6: ``:mongodb_cr``
 
    * - ``:auth_mech_properties``
      - Provides additional authentication mechanism properties.
@@ -414,7 +420,7 @@ Ruby Options
      - An array of OpenSSL::X509::Certificate representing the certificate authority certificates used to
        validate certs passed from the other end of the connection. One of :ssl_ca_cert, :ssl_ca_cert_string or
        :ssl_ca_cert_object (in order of priority) is required for :ssl_verify.
-     - ``Array<OpenSSL::X509::Certificate>``
+     - ``Array< OpenSSL::X509::Certificate >``
      - none
 
    * - ``:ssl_ca_cert_string``