doc: add a release note about proper `sourceRoot` usage

nixos/doc/manual/release-notes/rl-2311.section.md

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 ## Nixpkgs internals {#sec-release-23.11-nixpkgs-internals}
 
+- The use of `sourceRoot = "source";`, `sourceRoot = "source/subdir";`, and similar lines in package derivations using the default `unpackPhase` is deprecated as it requires `unpackPhase` to always produce a directory named "source". Use `sourceRoot = src.name`, `sourceRoot = "${src.name}/subdir";`, or `setSourceRoot = "sourceRoot=$(echo */subdir)";` or similar instead.
+
 - The `qemu-vm.nix` module by default now identifies block devices via
   persistent names available in `/dev/disk/by-*`. Because the rootDevice is
   identfied by its filesystem label, it needs to be formatted before the VM is