media: v4l2-fwnode: Document new usage patterns of v4l2_fwnode_endpoint_parse

Document that it is possible to provide defaults for multiple bus types to
v4l2_fwnode_endpoint_parse and v4l2_fwnode_endpoint_alloc_parse. Also
underline the fact that detecting the bus type without bus-type property
is only for the old drivers.

Also correct capitalisation of BT.656.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>