doc: ieee802154: driver: improved docs

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

Apart from extending and clarifying many details of the driver API, this
change also specifies certain formal properties of API calls, such as
"sleeps" or "isr-ok". Plus it removes semantic ambivalence that had been
resolved and hidden in existing implementation details but is highly
relevant to the way the API can be used and shall be implemented.

As far as possible this change introduces formal requirements language
(MAY, SHALL/MUST, SHOULD) to specify driver implementation requirements.

This change also introduces two definitions - for the moment being of
purely documentary relevance (i.e. to be referred in the documentation):
* IEEE802154_CONFIG_RX_SLOT_NONE
* IEEE802154_CONFIG_RX_SLOT_OFF

These "magic" values are required to clarify the exact semantics of RX
slots as to receiver activity.

Signed-off-by: Florian Grandel <[email protected]>

Author: fg-cfh