This PR attempts to fix publicsuffix/list#2241

Updating documentation to:

Software using the Public Suffix List will be able to determine cookie inheritance boundaries between domains, preventing cookies set on one domain from being accessible to other domains under the same public suffix. This protects users from cross-domain cookies setting while still allowing individual domains to set their own cookies.

Observations from Chromium and Firefox:

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.