YARN-7361. Improve the docker container runtime documentation. Contributed by Shane Kumpf

(cherry picked from commit fac72eef23bb0a74a34f289dd6ef50ffa4303aa4)

Conflicts:
	hadoop-yarn-project/hadoop-yarn/hadoop-yarn-site/src/site/markdown/DockerContainers.md

(cherry picked from commit d26281556406e53f9694ec2c54172d836b80df56)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-site/src/site/markdown/DockerContainers.md

@@ -144,7 +144,8 @@ The following properties should be set in yarn-site.xml:
       Optional. This configuration setting determines the capabilities
       assigned to docker containers when they are launched. While these may not
       be case-sensitive from a docker perspective, it is best to keep these
-      uppercase.
+      uppercase. To run without any capabilites, set this value to
+      "none" or "NONE"
     </description>
   </property>
 </configuration>
@@ -315,4 +316,4 @@ To run a Spark shell in Docker containers, run the following command:
 ```
 
 Note that the application master and executors are configured
-independently. In this example, we are using the hadoop-docker image for both.
+independently. In this example, we are using the hadoop-docker image for both.