[aws] Add mention of NLB and ALB to lb_target_groups docs
[aws] Add mention of NLB and ALB to lb_target_groups docs

[#157493693](https://www.pivotaltracker.com/story/show/157493693)

Co-authored-by: Belinda Liu <496afffef642eb3807bf00ef2898c80e8c6b937c@pivotal.io>

cloudfoundry/docs-bosh