[lldb][ELF] Add a comment to explain address class map type

It was pointed out that ordering is crucial here, so note that.

I also looked into using a vector instead, as described in
https://llvm.org/docs/ProgrammersManual.html#dss-sortedvectorset.

Which this is in theory perfect for, but we have at least 2 places
that update the map and both would need to sort/unique each time.
Plus this code is pretty bug prone.

If there is future refactoring it's one thing to consider.