Clarify that Kotlin DSL needs an import

Closes gh-13092

docs/modules/ROOT/pages/reactive/configuration/webflux.adoc

@@ -94,6 +94,8 @@ public class HelloWebfluxSecurityConfig {
 .Kotlin
 [source,kotlin,role="secondary"]
 -----
+import org.springframework.security.config.web.server.invoke
+
 @Configuration
 @EnableWebFluxSecurity
 class HelloWebfluxSecurityConfig {
@@ -122,6 +124,9 @@ class HelloWebfluxSecurityConfig {
 -----
 ====
 
+[NOTE]
+Make sure that you import the `invoke` function in your Kotlin class, sometimes the IDE will not auto-import it causing compilation issues.
+
 This configuration explicitly sets up all the same things as our minimal configuration.
 From here you can easily make the changes to the defaults.
 
@@ -176,6 +181,8 @@ static class MultiSecurityHttpConfig {
 .Kotlin
 [source,kotlin,role="secondary"]
 ----
+import org.springframework.security.config.web.server.invoke
+
 @Configuration
 @EnableWebFluxSecurity
 open class MultiSecurityHttpConfig {