Update documentation about WebTestClient Kotlin issue

Issue: SPR-16057

src/docs/asciidoc/languages/kotlin.adoc

@@ -654,6 +654,17 @@ class SpecificationLikeTests {
 ----
 
 
+[[kotlin-webtestclient-issue]]
+==== `WebTestClient` type inference issue in Kotlin
+
+`WebTestClient` is not usable yet in Kotlin due to a
+https://youtrack.jetbrains.com/issue/KT-5464[type inference issue] which is
+expected to be fixed as of Kotlin 1.3. You can watch
+https://jira.spring.io/browse/SPR-16057[SPR-16057] for up-to-date information. Meanwhile,
+the proposed alternative is to use directly `WebClient` with its Reactor and Spring Kotlin
+extensions to perform integration tests on an embedded WebFlux server.
+
+
 
 
 [[kotlin-getting-started]]
@@ -729,6 +740,7 @@ Here is a list of pending issues related to Spring + Kotlin support.
 
 ==== Spring Framework
 
+* https://jira.spring.io/browse/SPR-16057[Unable to use WebTestClient with mock server in Kotlin]
 * https://jira.spring.io/browse/SPR-15413[Add support for Kotlin coroutines]
 
 

src/docs/asciidoc/testing-webtestclient.adoc

@@ -8,6 +8,10 @@ to any server over an HTTP connection. It can also bind directly to WebFlux appl
 with <<testing.adoc#mock-objects-web-reactive,mock request and response>> objects,
 without the need for an HTTP server.
 
+[NOTE]
+====
+`WebTestClient` is not usable yet in Kotlin, see <<languages.adoc#kotlin-webtestclient-issue, this section>> for more details.
+====