When a handler method is annotated with `@ResponseStatus(reason="...")`,
Spring MVC will use `HttpServletResponse#sendError` to complete the
response. As a result, the Servlet container will send an HTML error
page and any existing data in the response buffer will be cleared.
This commit clarifies the `@ResponseStatus` Javadoc and ensures that a
message is logged at the WARN level if a handler method is annotated
like this and still returns a non-Void value. In this case, the return
value will be ignored and developers should be aware of this fact.
See gh-31113
Closes gh-31121
94a49c4ed3