VNET_DOMAIN_SET is not mentioned in documentation

[RăduicăIulian]

I searched more details in documentation about VNET_DOMAIN_SET in the 
following places:

-searched for it using "man -k VNET_DOMAIN_SET"

-searched in VNET(9) and in DOMAIN(9)

In the end, the only place that provided me the information required was 
sys/sys/domain.h line 88.

Is this a bug and I should fill a PR or there is another place where I 
should searched for an lighter explanation of this macro?