Document Welcome Page support ordering

This commit documents the relative ordering of `HandlerMapping` support
in Spring MVC and WebFlux applications.
As of Spring Framework 6.1.0, the Welcome Page support acts as a
fallback in case no index route has been defined by the application as a
`RouterFunction` or within an annotated `@Controller`.

Closes gh-34846

spring-boot-project/spring-boot-docs/src/docs/asciidoc/web/reactive.adoc

@@ -120,6 +120,21 @@ It first looks for an `index.html` file in the configured static content locatio
 If one is not found, it then looks for an `index` template.
 If either is found, it is automatically used as the welcome page of the application.
 
+This only acts as a fallback for actual index routes defined by the application.
+The ordering is defined by the order of `HandlerMapping` beans which is by default the following:
+
+[cols="1,1"]
+|===
+|`RouterFunctionMapping`
+|Endpoints declared with `RouterFunction` beans
+
+|`RequestMappingHandlerMapping`
+|Endpoints declared in `@Controller` beans
+
+|`RouterFunctionMapping` for the Welcome Page
+|The welcome page support
+|===
+
 
 
 [[web.reactive.webflux.template-engines]]

spring-boot-project/spring-boot-docs/src/docs/asciidoc/web/servlet.adoc

@@ -196,6 +196,20 @@ It first looks for an `index.html` file in the configured static content locatio
 If one is not found, it then looks for an `index` template.
 If either is found, it is automatically used as the welcome page of the application.
 
+This only acts as a fallback for actual index routes defined by the application.
+The ordering is defined by the order of `HandlerMapping` beans which is by default the following:
+
+[cols="1,1"]
+|===
+|`RouterFunctionMapping`
+|Endpoints declared with `RouterFunction` beans
+
+|`RequestMappingHandlerMapping`
+|Endpoints declared in `@Controller` beans
+
+|`WelcomePageHandlerMapping`
+|The welcome page support
+|===
 
 
 [[web.servlet.spring-mvc.favicon]]