docs(oauth) add config.auth_header_name parameter

See Kong/kong#2928

app/plugins/oauth2-authentication.md

@@ -75,6 +75,7 @@ form parameter                                    | default | description
 `config.enable_client_credentials`<br>*optional*   | `false` | An optional boolean value to enable the Client Credentials Grant flow ([RFC 6742 Section 4.4][client-credentials])
 `config.enable_implicit_grant`<br>*optional*       | `false` | An optional boolean value to enable the Implicit Grant flow which allows to provision a token as a result of the authorization process ([RFC 6742 Section 4.2][implicit-grant])
 `config.enable_password_grant`<br>*optional*       | `false` | An optional boolean value to enable the Resource Owner Password Credentials Grant flow ([RFC 6742 Section 4.3][password-grant])
+`config.auth_header_name`<br>*optional*            | `authorization` | The name of the header supposed to carry the access token. Default: `authorization`.
 `config.hide_credentials`<br>*optional*            | `false` | An optional boolean value telling the plugin to hide the credential to the upstream API server. It will be removed by Kong before proxying the request
 `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`.