Add release note for non-flake inputs having sourceInfo

Author: MattSturgeon

doc/manual/rl-next/outpath-and-sourceinfo-fixes.md

@@ -0,0 +1,15 @@
+---
+synopsis: Non-flake inputs now contain a `sourceInfo` attribute
+issues: 13164
+prs: 13170
+---
+
+Flakes have always a `sourceInfo` attribute which describes the source of the flake.
+The `sourceInfo.outPath` is often identical to the flake's `outPath`, however it can differ when the flake is located in a subdirectory of its source.
+
+Non-flake inputs (i.e. inputs with `flake = false`) can also be located at some path _within_ a wider source.
+This usually happens when defining a relative path input within the same source as the parent flake, e.g. `inputs.foo.url = ./some-file.nix`.
+Such relative inputs will now inherit their parent's `sourceInfo`.
+
+This iterates on the work done in 2.26 to improve relative path support ([#10089](https://github.com/NixOS/nix/pull/10089)),
+and resolves a regression introduced in 2.28 relating to nested relative path inputs ([#13164](https://github.com/NixOS/nix/issues/13164)).