documentation: Document which yielding methods are safe to call concurrently

documentation: Document which yielding methods are safe to call concurrently

This is a follow-up commit to ce55fa2bf2f5f8cf95532da585d835bafeeb3347.
I went through all methods in folks which yield to another async method,
and tried to work out whether the caller was safe to run multiple times
concurrently (e.g. begin a second asynchronous call to it between a previous
async call beginning and finishing).

I’ve marked all such methods as safe (or not safe) as appropriate. I
haven’t made any attempt to make the unsafe methods safe, except in one
case in backend-store.vala.