Clarify selectel_vpc_subnet_v2 resource documentation

[mipo256]

In the documentations page for the selectel_vpc_subnet_v2 resource, the attributes reference contains the network_id. But it is by no means clear what is the value of this network_id.

It is supposed to be the network where the public subnet is created, and, I guess, it will be the network_id of the external network, but I am not sure.

Can you please clarify this in documentation? In particular, it will be great to document:

1. Inside what network the created subnet will reside?
2. Why we are not allowed to specify the netowrk_id fin the resource block ourselves?