MXS-2824: Document basic user privileges

The documentation was not clear on what the difference between admin and
basic accounts is.

maxctrl/lib/create.js

@@ -319,7 +319,8 @@ exports.builder = function(yargs) {
             return yargs.epilog('The created user can be used with the MaxScale REST API as ' +
                                 'well as the MaxAdmin network interface. By default the created ' +
                                 'user will have read-only privileges. To make the user an ' +
-                                'administrative user, use the `--type=admin` option.')
+                                'administrative user, use the `--type=admin` option. ' +
+                                'Basic users can only perform `list` and `show` commands.')
                 .usage('Usage: create user <name> <password>')
         }, function(argv) {