Add description for "--cpu-shares=0".

Signed-off-by: Yuan Sun <sunyuan3@huawei.com>

docs/reference/run.md

@@ -800,7 +800,8 @@ can be modified by changing the container's CPU share weighting relative
 to the weighting of all other running containers.
 
 To modify the proportion from the default of 1024, use the `-c` or `--cpu-shares`
-flag to set the weighting to 2 or higher.
+flag to set the weighting to 2 or higher. If 0 is set, the system will ignore the
+value and use the default of 1024.
 
 The proportion will only apply when CPU-intensive processes are running.
 When tasks in one container are idle, other containers can use the