doc: ieee802154: l2: improved docs

Hides types used in the IEEE 802.15.4 L2 sub-API and L2-internal APIs
that are of no public interest and improves documentation of the
remainder.

The changes are mostly minor as the API documentation had already been
improved and clarified in previous changes. Also includes non-visible
documentation to the subsystem-internal Frame API by adding references
to the specification.

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