Document --serverinfo two-letter ISO country code support

[hoffie]

jamulussoftware/jamulus#2841 should be documented on the website.
We should make the ISO-code-based variant the preferred one and only keep the documentation for "Jamulus 3.9.0 or earlier".

Note: This is nice-to-have for 3.9.1, but in no way mandatory as it doesn't break existing documentation in any way.

[pljones]

Successfully merging a pull request may close this issue.

... or it may not. It should have, so I'll do it manually.