Breaking changes announcement for unstable

[waffle8946]

@eblechschmidt

Especially as the error message is not helping either:

error: evaluation aborted with the following error message: '(t.merge.v2 defs).value must only be accessed when .headError == null. This is a bug in code that consumes a module system type

This error will be improved in lib/types: fix either lazy value regression in case of non-matching v… by hsjobeki · Pull Request #438558 · NixOS/nixpkgs · GitHub