Thanks Xiaobing Zhou. We should document the format of this setting with examples. AccessControlList.java has a good description that we can reuse.
* The String is a a comma separated list of users and groups.
* The user list comes first and is separated by a space followed
* by the group list. For e.g. "user1,user2 group1,group2"
We can add that both users and groups are optional. So "user1", " group1", "", "user1 group1", "user1,user2 group1,group2" are all valid (note the leading space in " group1").
Also for dfs.permissions.supergroup, we should document that it accepts a single group name as the value. I have seen some administrators try to incorrectly configure this as a comma separated list of groups.