Add note for kubernetes executor services support

doc/ci/docker/using_docker_build.md

@@ -217,8 +217,8 @@ support this.
      # The 'docker' hostname is the alias of the service container as described at
      # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services.
      #
-     # Note that if you're using the Kubernetes executor, the variable
-     # should be set to tcp://localhost:2376 because of how the
+     # Note that if you're using GitLab Runner 12.7 or earlier with the Kubernetes executor and Kubernetes 1.6 or earlier,
+     # the variable must be set to tcp://localhost:2376 because of how the
      # Kubernetes executor connects services to the job container
      # DOCKER_HOST: tcp://localhost:2376
      #
@@ -279,12 +279,11 @@ variables:
   # The 'docker' hostname is the alias of the service container as described at
   # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services
   #
-  # Note that if you're using the Kubernetes executor, the variable should be set to
-  # tcp://localhost:2375 because of how the Kubernetes executor connects services
-  # to the job container
+  # Note that if you're using GitLab Runner 12.7 or earlier with the Kubernetes executor and Kubernetes 1.6 or earlier,
+  # the variable must be set to tcp://localhost:2375 because of how the
+  # Kubernetes executor connects services to the job container
   # DOCKER_HOST: tcp://localhost:2375
   #
-  # For non-Kubernetes executors, we use tcp://docker:2375
   DOCKER_HOST: tcp://docker:2375
   #
   # This will instruct Docker not to start over TLS.

doc/ci/docker/using_docker_images.md

@@ -345,6 +345,9 @@ For example, the following two definitions are equal:
 | `command`    | no       | 9.4 |Command or script that should be used as the container's command. It will be translated to arguments passed to Docker after the image's name. The syntax is similar to [`Dockerfile`'s `CMD`][cmd] directive, where each shell token is a separate string in the array. |
 | `alias`      | no       | 9.4 |Additional alias that can be used to access the service from the job's container. Read [Accessing the services](#accessing-the-services) for more information. |
 
+NOTE: **Note:**
+Alias support for the Kubernetes executor was [introduced](https://gitlab.com/gitlab-org/gitlab-runner/issues/2229) in GitLab Runner 12.8, and is only available for Kubernetes version 1.7 or later.
+
 ### Starting multiple services from the same image
 
 > Introduced in GitLab and GitLab Runner 9.4. Read more about the [extended