iio: buffer: add missing descriptions in iio_buffer_access_funcs

The members buffer_group and attrs of iio_buffer_access_funcs have no
descriptions for the documentation. Adding them.

Fixes: 08e7e0ad ("iio: buffer: Allocate standard attributes in the core")
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>