Add documentation on mkdir about seclabel

  group. If not provided, the directory is created with permissions 755 and

  owned by the root user and root group. If provided, the mode, owner and group

  will be updated if the directory exists already.

  If the directory does not exist, it will receive the security context from

  the current SELinux policy or its parent if not specified in the policy. If

  the directory exists, its security context will not be changed (even if

  different from the policy).

  > _action_ can be one of:

  * `None`: take no encryption action; directory will be encrypted if parent is.