Improve vSphere network docs

**What would you like to be added**:
Improve documentation around `network` for vsphere configuration and how only one single network is supported.
**Why is this needed**:
It's not clear that only one single network is supported. This can be confusing for users wanting to setup multi-network deployments, like HA across multiple datacenters.

Locate the vSphere configuration documentation and start with the section describing the network setting. Update it to state clearly that only one network is supported and explain the implication for multi-network deployments; done means users can understand this limitation before planning HA across multiple datacenters.

Written by the indexing model from the issue text.