Recommend to use private IPs for intra-region traffic

[cbrownstein-lambda]

          I'd probably skip the private vs public comparison, the difference is negligent, though I'd mention a recommendation to use private IPs for instance communication inside the same region and public IPs for instances in different regions.

_Originally posted by @achurak in https://github.com/LambdaLabs/lambda-docs/pull/147#discussion_r1060143760_