Documentation: builtins.fetchGit when used on a local path (#7706)

* Documentation:  builtins.fetchGit when used on a local path

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libexpr/primops/fetchTree.cc

@@ -456,6 +456,17 @@ static RegisterPrimOp primop_fetchGit({
           > **Note**
           >
           > This behavior is disabled in *Pure evaluation mode*.
+
+        - To fetch the content of a checked-out work directory:
+
+          ```nix
+          builtins.fetchGit ./work-dir
+          ```
+
+	  If the URL points to a local directory, and no `ref` or `rev` is
+	  given, `fetchGit` will use the current content of the checked-out
+	  files, even if they are not committed or added to Git's index. It will
+	  only consider files added to the Git repository, as listed by `git ls-files`.
     )",
     .fun = prim_fetchGit,
 });