[ci skip] Improve enable_starttls_auto documentation.

actionmailer/lib/action_mailer/base.rb

@@ -369,8 +369,8 @@ module ActionMailer
   #     This is a symbol and one of <tt>:plain</tt> (will send the password in the clear), <tt>:login</tt> (will
   #     send password Base64 encoded) or <tt>:cram_md5</tt> (combines a Challenge/Response mechanism to exchange
   #     information and a cryptographic Message Digest 5 algorithm to hash important information)
-  #   * <tt>:enable_starttls_auto</tt> - When set to true, detects if STARTTLS is enabled in your SMTP server
-  #     and starts to use it.
+  #   * <tt>:enable_starttls_auto</tt> - Detects if STARTTLS is enabled in your SMTP server and starts
+  #     to use it. Defaults to <tt>true</tt>.
   #   * <tt>:openssl_verify_mode</tt> - When using TLS, you can set how OpenSSL checks the certificate. This is
   #     really useful if you need to validate a self-signed and/or a wildcard certificate. You can use the name
   #     of an OpenSSL verify constant (<tt>'none'</tt>, <tt>'peer'</tt>, <tt>'client_once'</tt>,