net: ieee802154: Provide a kdoc to the address structure

Give this structure a header to better explain its content.

Signed-off-by: David Girault <david.girault@qorvo.com>
[miquel.raynal@bootlin.com: Isolate this change from a bigger commit and
                            reword the comment]
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220201180956.93581-1-miquel.raynal@bootlin.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>