Add Docs for Custom Bearer Token Header

Issue gh-8337

Author: jzheaux

docs/manual/src/docs/asciidoc/_includes/reactive/oauth2/resource-server.adoc

@@ -1077,6 +1077,29 @@ This approach allows us to add and remove elements from the repository (shown as
 NOTE: It would be unsafe to simply take any issuer and construct an `ReactiveAuthenticationManager` from it.
 The issuer should be one that the code can verify from a trusted source like a whitelist.
 
+[[webflux-oauth2resourceserver-bearertoken-resolver]]
+== Bearer Token Resolution
+
+By default, Resource Server looks for a bearer token in the `Authorization` header.
+This, however, can be customized.
+
+For example, you may have a need to read the bearer token from a custom header.
+To achieve this, you can wire an instance of `ServerBearerTokenAuthenticationConverter` into the DSL, as you can see in the following example:
+
+.Custom Bearer Token Header
+====
+.Java
+[source,java,role="primary"]
+----
+ServerBearerTokenAuthenticationConverter converter = new ServerBearerTokenAuthenticationConverter();
+converter.setBearerTokenHeaderName(HttpHeaders.PROXY_AUTHORIZATION);
+http
+    .oauth2ResourceServer(oauth2 -> oauth2
+        .bearerTokenConverter(converter)
+    );
+----
+====
+
 == Bearer Token Propagation
 
 Now that you're in possession of a bearer token, it might be handy to pass that to downstream services.

docs/manual/src/docs/asciidoc/_includes/servlet/oauth2/oauth2-resourceserver.adoc

@@ -1955,22 +1955,24 @@ However, if you resolve it by a claim in the bearer token, read on to learn abou
 === Bearer Token Resolution
 
 By default, Resource Server looks for a bearer token in the `Authorization` header.
-This, however, can be customized in a couple of ways.
+This, however, can be customized in a handful of ways.
 
 ==== Reading the Bearer Token from a Custom Header
 
 For example, you may have a need to read the bearer token from a custom header.
-To achieve this, you can wire a `HeaderBearerTokenResolver` instance into the DSL, as you can see in the following example:
+To achieve this, you can expose a `DefaultBearerTokenResolver` as a bean, or wire an instance into the DSL, as you can see in the following example:
 
 .Custom Bearer Token Header
 ====
 .Java
 [source,java,role="primary"]
 ----
-http
-    .oauth2ResourceServer(oauth2 -> oauth2
-        .bearerTokenResolver(new HeaderBearerTokenResolver("x-goog-iap-jwt-assertion"))
-    );
+@Bean
+BearerTokenResolver bearerTokenResolver() {
+    DefaultBearerTokenResolver bearerTokenResolver = new DefaultBearerTokenResolver();
+    bearerTokenResolver.setBearerTokenHeaderName(HttpHeaders.PROXY_AUTHORIZATION);
+    return bearerTokenResolver;
+}
 ----
 
 .Xml
@@ -1981,12 +1983,14 @@ http
 </http>
 
 <bean id="bearerTokenResolver"
-        class="org.springframework.security.oauth2.server.resource.web.HeaderBearerTokenResolver">
-    <constructor-arg value="x-goog-iap-jwt-assertion"/>
+        class="org.springframework.security.oauth2.server.resource.web.DefaultBearerTokenResolver">
+    <property name="bearerTokenHeaderName" value="Proxy-Authorization"/>
 </bean>
 ----
 ====
 
+Or, in circumstances where a provider is using both a custom header and value, you can use `HeaderBearerTokenResolver` instead.
+
 ==== Reading the Bearer Token from a Form Parameter
 
 Or, you may wish to read the token from a form parameter, which you can do by configuring the `DefaultBearerTokenResolver`, as you can see below: