media: v4l2-fwnode: Rework v4l2_fwnode_endpoint_parse documentation

Rework the documentation of v4l2_fwnode_endpoint_parse for better
readability, usefulness and correctness.

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