docs(oauth2) add config.refresh_token_ttl property

See Kong/kong#2942
From #535

Signed-off-by: Thibault Charbonnier <thibaultcha@me.com>

app/plugins/oauth2-authentication.md

@@ -80,8 +80,13 @@ form parameter                                    | default | description
 `config.accept_http_if_already_terminated`<br>*optional* | `false` | Accepts HTTPs requests that have already been terminated by a proxy or load balancer and the `x-forwarded-proto: https` header has been added to the request. Only enable this option if the Kong server cannot be publicly accessed and the only entry-point is such proxy or load balancer.
 `config.anonymous`<br>*optional*        | ``      | An optional string (consumer uuid) value to use as an "anonymous" consumer if authentication fails. If empty (default), the request will fail with an authentication failure `4xx`. Please note that this value must refer to the Consumer `id` attribute which is internal to Kong, and **not** its `custom_id`.
 `config.global_credentials`<br>*optional*   | `false` | An optional boolean value that allows to use the same OAuth credentials generated by the plugin with any other API whose OAuth 2.0 plugin configuration also has `config.global_credentials=true`.
+`config.refresh_token_ttl`<br>*optional*   | `1209600` | An optional integer value telling the plugin how many seconds a token/refresh token pair is valid for, and can be used to generate a new access token. Default value is 2 weeks. Set to `0` to keep the token/refresh token pair indefinitely valid.
 ----
 
+<div class="alert alert-warning">
+    <center>The option <code>config.refresh_token_ttl</code> is only available from version 0.12.0 and later</center>
+</div>
+
 ## Usage
 
 In order to use the plugin, you first need to create a consumer to associate one or more credentials to. The Consumer represents a developer using the final service/API.