Fixed #6327 -- Added has_module_permission method to BaseModelAdmin

* Added tests for this new method and for the other ones:
  * has_add_permission()
  * has_change_permission()
  * has_delete_permission()
* Updated the documentation
I wasn't expecting this additional check to be added in all these cases. In the default case, there's no change and it doesn't seem very DRY to require both.