[docs] Add IPv6 example for rate limit exceptions

[daenney]

Description

• Add IPv6 example for rate limit exceptions

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[mirabilos]

What? Why?

/64 is the smallest standard subnet that’s not a single host. End users get /48 normally, or /56 if the ISP is stingy.

I think supporting a /48 should be given.

[tsmethurst]

Open an issue instead of commenting on a (now closed) documentation PR from a week ago please. EDIT: The PR doesn't change the behavior, it just better documents what we were already doing. If you wanna discuss changing the behavior, a new issue is the place to do it.