Improve "[WireGuard] Header / MTU sizes for Wireguard"

[Yousong Zhou]

Hi,

For WireGuard overhead breakdown [1], maybe it's worth also mentioning
that N the length of encrypted data will be padded to be multiples of
16.

I am only aware of this when fragmentation was spotted.  With 1500 as
MTU for ethernet, PPPoE has MTU 1492 (1500 - 8).  I thought 1432 (1492
- 60) for wireguard should work for ipv4-only traffic. It needs to be
1424 to avoid fragmentation.

Google also directed me to an old deprecated link [2] with pointer to
[1].  So maybe it's useful this info should have a place in the
wireguard website.

 [1] https://lists.zx2c4.com/pipermail/wireguard/2017-December/002201.html
 [2] https://lists.zx2c4.com/pipermail/wireguard/2016-July/000314.html

Regards,
                yousong