net: l2: ieee802154: document non-standard channel page selection

The IEEE 802.15.4 standard does not specify a "Sub-GHz" option.
Therefore this option will be deprecated in the foreseeable future.

Rationale: Selecting PHYs and frequency bands is abstracted by the
concept of channel pages (see IEEE 802.15.4-2020, 10.1.3).

Radio drivers may expose additional configuration options that
specify the low level frequency band, operating mode and other
PHY-specific parameters. Such parameters should be exposed on a driver
instance level, e.g. via devicetree or as runtime options of the driver.
PHY-specific attributes derived from driver settings (e.g. available
channels) can then be announced to L2 w/o the user having to be aware of
any implementation details of the specific PHY.

Future changes will introduce the necessary infrastructure to define
channel pages and other PHY-specific configuration options to
replace this option. New drivers should therefore not rely on this
option.

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