Document custom HttpMessageConverter support for WebAuthn

Issue gh-16397

docs/modules/ROOT/pages/servlet/authentication/passkeys.adoc

@@ -70,6 +70,7 @@ SecurityFilterChain filterChain(HttpSecurity http) {
 			.allowedOrigins("https://example.com")
 			// optional properties
 			.creationOptionsRepository(new CustomPublicKeyCredentialCreationOptionsRepository())
+			.messageConverter(new CustomHttpMessageConverter())
 		);
 	return http.build();
 }
@@ -100,6 +101,7 @@ open fun filterChain(http: HttpSecurity): SecurityFilterChain {
 			allowedOrigins = setOf("https://example.com")
 			// optional properties
 			creationOptionsRepository = CustomPublicKeyCredentialCreationOptionsRepository()
+			messageConverter = CustomHttpMessageConverter()
 		}
 	}
 }

docs/modules/ROOT/pages/whats-new.adoc

@@ -17,4 +17,5 @@ Note that this may affect reports that operate on this key name.
 
 == WebAuthn
 
+* https://github.com/spring-projects/spring-security/pull/16397[gh-16397] - Added the ability to configure a custom `HttpMessageConverter` for Passkeys using the optional xref:servlet/authentication/passkeys.adoc#passkeys-configuration[`messageConverter` property] on the `webAuthn` DSL.
 * https://github.com/spring-projects/spring-security/pull/16396[gh-16396] - Added the ability to configure a custom xref:servlet/authentication/passkeys.adoc#passkeys-configuration-pkccor[`PublicKeyCredentialCreationOptionsRepository`]