[2.7] docker_container: userns_mode description updated to mention valid value (#51470)

* description updated to mention valid value

The only valid value for userns_mode on docker run command is "host". This should be mentioned in description because it is hard to find in docker documentation as well: https://github.com/docker/docker.github.io/issues/7071

(cherry picked from commit d9135f3960)

* Update lib/ansible/modules/cloud/docker/docker_container.py

Co-Authored-By: TimoKramer <4785848+TimoKramer@users.noreply.github.com>
(cherry picked from commit 27dc399885)

lib/ansible/modules/cloud/docker/docker_container.py

@@ -226,7 +226,7 @@ options:
       - Connect the container to a network. Choices are "bridge", "host", "none" or "container:<name|id>"
   userns_mode:
      description:
-       - User namespace to use
+       - Set the user namespace mode for the container. Currently, the only valid value is C(host).
      version_added: "2.5"
   networks:
      description: