doc: ieee802154: mgmt: improved docs

Hides types in the IEEE 802.15.4 net_mgmt sub-API that are of no public
interest and improves documentation of the remainder.

All constants that are not meant to be used by applications have been
hidden. Only the API actually being of relevance to application
developers is now publicly visible and documented.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>