net: ip: document endianness

To maintain POSIX compatibility, link-layer addresses are encoded in big
endian in the IP stack and socket API.

The intended endianness was however not documented everywhere which led
to bugs and inconsistencies. The IEEE 802.15.4 L2 stack, for example,
sometimes stores addresses in little endian in structures that intend
them to be stored in POSIX-compliant big endian byte order.

This change documents intended endianness within the realm of the
IP and sockets stack. Conversion bugs are fixed in a separate commit.

Signed-off-by: Florian Grandel <jerico.dev@gmail.com>