Document support for anonymized IP handling

## What are the schemas that are affected by the issue
xdm/context/environment.schema.json

## What are examples of products that are impacted by the issue
Adobe Analytics and Adobe Primetime.

The IP fields (v4 and v6) are strings with Json formats:
"format": "ipv4",
"format": "ipv6",

We see various strategies for removing the level of identification from IP addresses like
1.1.1.1 -> 1.1.1.x (customer example)
Adobe Analytics sets the last octet to zero ->1.1.1.0 via configuration.

The standard validations work with the latter 'zero' option, but not with the 'x' option. We need to document what we support in XDM.

For IPv6 more research is needed, but I expect the same general issues and solutions.

Start by reading xdm/context/environment.schema.json and inspect how the IPv4 and IPv6 fields currently validate. Document which anonymization forms XDM supports, including the IPv4 zeroed-octet and masked forms, and clarify the IPv6 position; done means users can determine whether their anonymized values are supported.

Written by the indexing model from the issue text.