Domain preparation (avoid "A-Label" in the domain names).

Nobody has claimed this yet.

According the IDNA (https://tools.ietf.org/html/rfc5890) there are a several ways to represent the same domain if we use non-ASCII symbols.
For example, the domain `öbb.at` can be represent by `öbb.at` or `xn--bb-eka.at`. The first form is called U-label and uses unicode symbols. The second form is called A-label and uses only ASCII-symbols.

To convert from an one form to the another uses the `Punycode` algorithm (https://tools.ietf.org/html/rfc3492, https://tools.ietf.org/html/rfc3490#section-4, https://www.unicode.org/reports/tr46/#Compatibility_Processing and online tool https://www.punycoder.com/).

In the specification (https://matrix.org/docs/spec/appendices.html) it is only says that `DNS names for use with Matrix should follow the conventional restrictions for internet hostnames: they should consist of a series of labels separated by ., where each label consists of the alphanumeric characters or hyphens.` So we can have hostnames with non-ASCII symbols.

Should we have a domain preparation step to avoid using the A-label like `xn--bb-eka.at` and always use only U-label like `öbb.at`?

Start with the linked Matrix specification and the IDNA, Punycode, and Unicode TR46 references cited in the issue. Determine whether Matrix should prefer U-labels over A-labels and identify the relevant specification section. Done means reaching a clear decision and documenting the agreed domain-preparation behavior.

Written by the indexing model from the issue text.