write docs about platformdirs change (borg2)

[ThomasWaldmann]

See #7300.

TODO: consider what happens at borg transfer time and what docs or code are needed for that, especially on macOS where the dirs will change in any case. On other OSes, dirs might change under some circumstances.

For keyfile (not: repokey) users, this might mean that the borg 1.x key can't be found.

[ThomasWaldmann]

the docs should also talk about typical dir locations on windows, macOS, linux.