Add ref doc for refresh_token grant

Fixes gh-7398

Author: jgrandja

docs/manual/src/docs/asciidoc/_includes/servlet/preface/oauth2-client.adoc

@@ -81,6 +81,7 @@ The following sections will go into more detail on the core components used by O
 ** <<oauth2Client-authorized-manager-provider, OAuth2AuthorizedClientManager / OAuth2AuthorizedClientProvider>>
 * <<oauth2Client-auth-grant-support>>
 ** <<oauth2Client-auth-code-grant, Authorization Code>>
+** <<oauth2Client-refresh-token-grant, Refresh Token>>
 ** <<oauth2Client-client-creds-grant, Client Credentials>>
 * <<oauth2Client-additional-features>>
 ** <<oauth2Client-registered-authorized-client, Resolving an Authorized Client>>
@@ -552,6 +553,80 @@ public class OAuth2ClientSecurityConfig extends WebSecurityConfigurerAdapter {
 ----
 
 
+[[oauth2Client-refresh-token-grant]]
+==== Refresh Token
+
+[NOTE]
+Please refer to the OAuth 2.0 Authorization Framework for further details on the https://tools.ietf.org/html/rfc6749#section-1.5[Refresh Token].
+
+
+===== Refreshing an Access Token
+
+[NOTE]
+Please refer to the https://tools.ietf.org/html/rfc6749#section-6[Access Token Request/Response] protocol flow for the Refresh Token grant.
+
+The default implementation of `OAuth2AccessTokenResponseClient` for the Refresh Token grant is `DefaultRefreshTokenTokenResponseClient`, which uses a `RestOperations` when refreshing an access token at the Authorization Server’s Token Endpoint.
+
+The `DefaultRefreshTokenTokenResponseClient` is quite flexible as it allows you to customize the pre-processing of the Token Request and/or post-handling of the Token Response.
+
+
+===== Customizing the Access Token Request
+
+If you need to customize the pre-processing of the Token Request, you can provide `DefaultRefreshTokenTokenResponseClient.setRequestEntityConverter()` with a custom `Converter<OAuth2RefreshTokenGrantRequest, RequestEntity<?>>`.
+The default implementation `OAuth2RefreshTokenGrantRequestEntityConverter` builds a `RequestEntity` representation of a standard https://tools.ietf.org/html/rfc6749#section-6[OAuth 2.0 Access Token Request].
+However, providing a custom `Converter`, would allow you to extend the standard Token Request and add custom parameter(s).
+
+IMPORTANT: The custom `Converter` must return a valid `RequestEntity` representation of an OAuth 2.0 Access Token Request that is understood by the intended OAuth 2.0 Provider.
+
+
+===== Customizing the Access Token Response
+
+On the other end, if you need to customize the post-handling of the Token Response, you will need to provide `DefaultRefreshTokenTokenResponseClient.setRestOperations()` with a custom configured `RestOperations`.
+The default `RestOperations` is configured as follows:
+
+[source,java]
+----
+RestTemplate restTemplate = new RestTemplate(Arrays.asList(
+		new FormHttpMessageConverter(),
+		new OAuth2AccessTokenResponseHttpMessageConverter()));
+
+restTemplate.setErrorHandler(new OAuth2ErrorResponseErrorHandler());
+----
+
+TIP: Spring MVC `FormHttpMessageConverter` is required as it's used when sending the OAuth 2.0 Access Token Request.
+
+`OAuth2AccessTokenResponseHttpMessageConverter` is a `HttpMessageConverter` for an OAuth 2.0 Access Token Response.
+You can provide `OAuth2AccessTokenResponseHttpMessageConverter.setTokenResponseConverter()` with a custom `Converter<Map<String, String>, OAuth2AccessTokenResponse>` that is used for converting the OAuth 2.0 Access Token Response parameters to an `OAuth2AccessTokenResponse`.
+
+`OAuth2ErrorResponseErrorHandler` is a `ResponseErrorHandler` that can handle an OAuth 2.0 Error, eg. 400 Bad Request.
+It uses an `OAuth2ErrorHttpMessageConverter` for converting the OAuth 2.0 Error parameters to an `OAuth2Error`.
+
+Whether you customize `DefaultRefreshTokenTokenResponseClient` or provide your own implementation of `OAuth2AccessTokenResponseClient`, you'll need to configure it as shown in the following example:
+
+[source,java]
+----
+// Customize
+OAuth2AccessTokenResponseClient<OAuth2RefreshTokenGrantRequest> refreshTokenTokenResponseClient = ...
+
+OAuth2AuthorizedClientProvider authorizedClientProvider =
+		OAuth2AuthorizedClientProviderBuilder.builder()
+				.authorizationCode()
+				.refreshToken(configurer -> configurer.accessTokenResponseClient(refreshTokenTokenResponseClient))
+				.build();
+
+...
+
+authorizedClientManager.setAuthorizedClientProvider(authorizedClientProvider);
+----
+
+[NOTE]
+`OAuth2AuthorizedClientProviderBuilder.builder().refreshToken()` configures a `RefreshTokenOAuth2AuthorizedClientProvider`,
+which is an implementation of an `OAuth2AuthorizedClientProvider` for the Refresh Token grant.
+
+The `OAuth2RefreshToken` may optionally be returned in the Access Token Response for the `authorization_code` and `password` grant types.
+If the `OAuth2AuthorizedClient.getRefreshToken()` is available and the `OAuth2AuthorizedClient.getAccessToken()` is expired, it will automatically be refreshed by the `RefreshTokenOAuth2AuthorizedClientProvider`.
+
+
 [[oauth2Client-client-creds-grant]]
 ==== Client Credentials