net: l2: ieee802154: improve inline documentation

Over time, some non-standard concepts and extensions were introduced
into the stack (in KConfig, in drivers, in the internal API and into the
implementation) which makes introduction of additional standard-
compliant extensions like TSCH (and others) unnecessarily difficult.

To introduce extensions like TSCH it is required for the IEEE 802.15.4
stack to become more structurally aligned with the standard again which
will be the focus of some of the upcoming preparatory changes.

One way to check and prove standard compliance is to reference the
standard from within the source code. This change therefore introduces
inline references to the IEEE 802.15.4-2020 standard wherever possible.
Deviations from the standard are documented with TODO or deprecation
labels to be addressed in future changes.

In the future, new code introduced to the IEEE 802.15.4 stack should
be documented and reviewed for standard-compliance to avoid further
divergence. Most importantly:
* MAC/PHY configuration (via net mgmt, radio API, devicetree or
  KConfig) should always be directly linked to well-defined MAC/PHY
  PIB attributes if visible to the MAC API or the end user.
* Net management/shell/radio API commands should have a documented
  reference to the corresponding MLME action from the standard.

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