media: doc-rst: ipu3: clarification on data type conversion of IEFD CU

The data type conversion of the IEFD CU inputs in ipu3 uapi
is ambiguities, add some clarification to help user to
understand this conversion.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Suggested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Tomasz Figa <tfiga@chromium.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>