doc: net: ptp: PTP structs

Improves documentation of PTP structs and explains basic underlying
concepts to increase the probability that these structs will be used
correctly and consistently.

Also introduces references to the underlying specifications.

Note: We currently (ab)use the PTP structs for timestamps in the IEEE
802.15.4 context for which they are undefined. It is also not ideal that
the generic `struct net_pkt` depends directly on PTP. Future changes
will therefore have to remove the reference to PTP structs in net_pkt
and replace them by net_time_t. Clients will then have to convert these
to PTP structures if required.

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