Documentation/cxl: Add bus internal docs

Kernel docs are already present in this file, but nothing is instructed
to generate them. Address that.

Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/20210913163324.1008564-2-ben.widawsky@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>