Add some more use cases of IP addresses from PAT

[sysrqb]

The Private Access Tokens draft highlights a couple use cases we don't quite capture in the current text: https://datatracker.ietf.org/doc/html/draft-private-access-tokens-01.txt#section-1.4

Two are:

• limit the amount of content an IP address can access over a given time period (referred to as a "metered paywall")
• rate-limit access from an IP address to prevent fraud and abuse

We already include anti-abuse usage, but we don't mention "rate-limiting" - that seems useful.