#9365 introduced the ability to update ERFA leap seconds. A few updates would still be nice

• Document it in a bit more detail, in particular to tell how to avoid downloading with iers.conf.auto_download = False (both in iers docs and in LeapSeconds docstring).
• Document how it can be used to simulate having missing leap seconds (@eteq).
• Make the various URLs lists that include mirrors (easier with Cache preload #9182 merged).
• Hard-code system files for more various operating systems.

System leap second file locations:

• Debian/Ubuntu: '/usr/share/zoneinfo/leap-seconds.list'