lib: packing: document recently added APIs

Extend the documentation for the packing library, covering the intended use
for the recently added APIs. This includes the pack() and unpack() macros,
as well as the pack_fields() and unpack_fields() macros.

Add a note that the packing() API is now deprecated in favor of pack() and
unpack().

For the pack_fields() and unpack_fields() APIs, explain the rationale for
when a driver may want to select this API. Provide an example which shows
how to define the fields and call the pack_fields() and unpack_fields()
macros.

Co-developed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20241210-packing-pack-fields-and-ice-implementation-v10-4-ee56a47479ac@intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>