Update documentation about Kubernetes executor authentication

doc/ci/docker/using_docker_images.md

@@ -545,9 +545,8 @@ runtime.
   support for using private registries, which required manual configuration
   of credentials on runner's host. We recommend to upgrade your Runner to
   at least version **1.8** if you want to use private registries.
-- Not available for [Kubernetes executor](https://docs.gitlab.com/runner/executors/kubernetes.html),
-  follow <https://gitlab.com/gitlab-org/gitlab-runner/-/issues/2673> for
-  details.
+- Available for [Kubernetes executor](https://docs.gitlab.com/runner/executors/kubernetes.html)
+  in GitLab Runner 13.1 and later.
 
 ### Using statically-defined credentials
 
@@ -601,6 +600,7 @@ There are two ways to determine the value of `DOCKER_AUTH_CONFIG`:
   Open a terminal and execute the following command:
 
   ```shell
+  # Note the use of "-n" - it prevents encoding a newline in the password.
   echo -n "my_username:my_password" | base64
 
   # Example output to copy