doc: linux-wpan: Fulfill the description of missed 802.15.4 APIs

There are more functions and operations which must be used or implemented
in each IEEE 802.15.4 device driver, but are not mentioned in the Device
drivers API section of Documentation/networking/ieee802154.txt.  Therefore,
I want to fulfill the missed part into the documentation with this patch.

Signed-off-by: Jian-Hong Pan <starnight@g.ncu.edu.tw>
Acked-by: Alexander Aring <aring@mojatatu.com>
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>