net: doc: Add missing doxygen comments to structs

The doxygen comment for ieee802154_radio_api, net_l2 and
net_pkt structs was missing, which meant that no documentation
was generated for them.

Also add comment to "enum net_l2_flags" as that looks better in
the generated html file.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
3 files changed