Spell out in the Admin API docs which ones are incompatible with MSC3861

[Half-Shot]

I keep looking back at #15582 to remember which API routes are disabled, but that's not helpful for people with less context than me. Even if it's an experimental thing, I think pragmatically enough people will try to use this (with Element X requiring it for delegated auth) that we should at least mention in which cases these APIs are disabled.