Improve invalid Content-Type handling in WebFlux

Closes gh-29565

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -984,7 +984,8 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	/**
 	 * Return the {@linkplain MediaType media type} of the body, as specified
 	 * by the {@code Content-Type} header.
-	 * <p>Returns {@code null} when the content-type is unknown.
+	 * <p>Returns {@code null} when the {@code Content-Type} header is not set.
+	 * @throws InvalidMediaTypeException if the media type value cannot be parsed
 	 */
 	@Nullable
 	public MediaType getContentType() {