Add clarifications about different proxy settings

[bo-quan]

We should not make ngcacher_proxy as always required since the install works without needing a ngcacher_proxy if the nodes have Internet access. We should add a comment in the install guide to indicate ngcacher_proxy should be required if the nodes are behind a proxy; also add some clarification that ngcacher_proxy is normally different from http_proxy.

[bo-quan]

We also want to add explanations about newly added NODE_PROXY section.