staging: comedi: comedidev.h: document struct comedi_buf_map

Add kernel-doc for `struct comedi_buf_map` and the associated `struct
comedi_buf_page`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>