Update cookies documentation

[groundcat]

Attempting to address publicsuffix/list#2241

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For experimental purpose, I have just set up a cookie tester at https://secaas.hk (secaas.hk is currently listed in the PSL) to test cookie behavior in Firefox and Chromium,

Observations:

1. We can indeed set cookies on a public suffix (suffix.tld)
2. As expected, subdomain1.suffix.tld and subdomain2.suffix.tld cannot set cookies on each other.
3. As expected, subdomain1.suffix.tld can set cookies on sub.subdomain1.suffix.tld.
4. As expected, subdomain1.suffix.tld and suffix.tld cannot set cookies on each other.

The documentation's current wording suggests that the PSL prevents cookie-setting entirely on public suffixes, when in reality it only prevents cookie sharing/inheritance between separate registrable domains under that public suffix. This is an important distinction that the documentation fails to make clear. A more accurate description would explain that the PSL helps prevent cookies set on one domain from being accessible to other domains under the same public suffix.