Improve documentation on reading form data

`@RequestParam` should be favored over `@RequestBody`
which can't always be used reliably due to how the Servlet
API behaves.

Closes spring-projectsgh-33409

Author: sdeleuze

framework-docs/modules/ROOT/pages/web/webmvc/mvc-controller/ann-methods/requestbody.adoc

@@ -34,6 +34,10 @@ Kotlin::
 You can use the xref:web/webmvc/mvc-config/message-converters.adoc[Message Converters] option of the xref:web/webmvc/mvc-config.adoc[MVC Config] to
 configure or customize message conversion.
 
+NOTE: Form data should be read using xref:web/webmvc/mvc-controller/ann-methods/requestparam.adoc[`@RequestParam`],
+not with `@RequestBody` which can't always be used reliably since in the Servlet API, request parameter
+access causes the request body to be parsed, and it can't be read again.
+
 You can use `@RequestBody` in combination with `jakarta.validation.Valid` or Spring's
 `@Validated` annotation, both of which cause Standard Bean Validation to be applied.
 By default, validation errors cause a `MethodArgumentNotValidException`, which is turned

framework-docs/modules/ROOT/pages/web/webmvc/mvc-controller/ann-methods/requestparam.adoc

@@ -72,6 +72,48 @@ values for the same parameter name.
 When an `@RequestParam` annotation is declared as a `Map<String, String>` or
 `MultiValueMap<String, String>`, without a parameter name specified in the annotation,
 then the map is populated with the request parameter values for each given parameter name.
+The following example shows how to do so with form data processing:
+
+[tabs]
+======
+Java::
++
+[source,java,indent=0,subs="verbatim,quotes",role="primary"]
+----
+	@Controller
+	@RequestMapping("/pets")
+	class EditPetForm {
+
+		// ...
+
+		@PostMapping(value = "/process", consumes = MediaType.APPLICATION_FORM_URLENCODED_VALUE)
+		public String processForm(@RequestParam MultiValueMap<String, String> params) {
+			// ...
+		}
+
+		// ...
+	}
+----
+Kotlin::
++
+[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
+----
+	@Controller
+	@RequestMapping("/pets")
+	class EditPetForm {
+
+		// ...
+
+		@PostMapping("/process", consumes = [MediaType.APPLICATION_FORM_URLENCODED_VALUE])
+		fun processForm(@RequestParam params: MultiValueMap<String, String>): String {
+			// ...
+		}
+
+		// ...
+
+	}
+----
+======
 
 Note that use of `@RequestParam` is optional (for example, to set its attributes).
 By default, any argument that is a simple value type (as determined by