Document path values in inputs

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/nix/flake.md

@@ -212,6 +212,13 @@ Currently the `type` attribute can be one of the following:
     root of a tree can use `path:./foo` to access the flake in
     subdirectory `foo`, but `path:../bar` is illegal.
 
+Path inputs can be specified with path values in `flake.nix`. Path values are a syntax for `path` inputs, and they are converted by
+1. resolving them into relative paths, relative to the base directory of `flake.nix`
+2. escaping URL characters (refer to IETF RFC?)
+3. prepending `path:`
+
+Note that the allowed syntax for path values in flake `inputs` may be more restrictive than general Nix, so you may need to use `path:` if your path contains certain special characters. See [Path literals](@docroot@/language/syntax#path-literal)
+
   Note that if you omit `path:`, relative paths must start with `.` to
   avoid ambiguity with registry lookups (e.g. `nixpkgs` is a registry
   lookup; `./nixpkgs` is a relative path).