ipv6 docs are out of date

[cjcjameson]

Last update to https://github.com/cloudfoundry/docs-cf-admin/blob/master/enabling_ipv6.html.md.erb was in November 2016 and since then AWS has made some big improvements https://aws.amazon.com/blogs/aws/new-ipv6-support-for-ec2-instances-in-virtual-private-clouds/

Probably other IaaSes have as well. What's the latest Cloud Foundry recommendation on this? I assume it's a moderately fast-moving space, so not making a pull request in deference to those who know.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/150811932

The labels on this github issue will be updated when the story is started.