It looks like the good doc on permissions is the HDFS Permissions Guide. The HDFS Permissions Guide link changed or is obsolete, but a google search with “hdfs permissions guide” and you should find a good reference.
My simple view is that all the user permissions work the same as unix, with out all the stickyness (no sticky bits etc.). For the group permissions, the supergroup is shown; and this belongs the the owner of the namenode process.
Seems pretty direct.