Add @RequestMapping produces documentation about charset

Issue: SPR-13600

Author: sdeleuze

spring-web/src/main/java/org/springframework/web/bind/annotation/RequestMapping.java

@@ -414,8 +414,11 @@
 	 * <pre class="code">
 	 * produces = "text/plain"
 	 * produces = {"text/plain", "application/*"}
+	 * produces = "application/json; charset=UTF-8"
 	 * </pre>
-	 * Expressions can be negated by using the "!" operator, as in "!text/plain", which matches
+	 * <p>It affects the actual content type written, for example to produce a JSON response
+	 * with UTF-8 encoding, {@code "application/json; charset=UTF-8"} should be used.
+	 * <p>Expressions can be negated by using the "!" operator, as in "!text/plain", which matches
 	 * all requests with a {@code Accept} other than "text/plain".
 	 * <p><b>Supported at the type level as well as at the method level!</b>
 	 * When used at the type level, all method-level mappings override

src/asciidoc/web-mvc.adoc

@@ -1028,13 +1028,21 @@ condition. For example:
 [subs="verbatim,quotes"]
 ----
 	@Controller
-	@RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET, **produces="application/json"**)
+	@RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET, **produces="application/json; charset=UTF-8"**)
 	@ResponseBody
 	public Pet getPet(@PathVariable String petId, Model model) {
 		// implementation omitted
 	}
 ----
 
+[NOTE]
+====
+Be aware that the media type specified in the __produces__ condition can also optionally
+specify a character set. For example, in the code snippet above we specify the same media
+type than the default one configured in `MappingJackson2HttpMessageConverter`, including
+the `UTF-8` charset.
+====
+
 Just like with __consumes__, producible media type expressions can be negated as in
 __!text/plain__ to match to all requests other than those with an __Accept__ header
 value of __text/plain__.
@@ -3847,7 +3855,7 @@ When writing error information, the status code and the error message set on the
 	@Controller
 	public class ErrorController {
 
-		@RequestMapping(path="/error", produces="application/json")
+		@RequestMapping(path="/error", produces="application/json; charset=UTF-8")
 		@ResponseBody
 		public Map<String, Object> handle(HttpServletRequest request) {