rl-next: Add *general* note about git fetcher reimpl

doc/manual/rl-next/git-fetcher.md

@@ -0,0 +1,12 @@
+---
+synopsis: Git fetching has been reimplemented
+prs: 9240 9241 9258 9480
+---
+
+Nix can fetch sources from Git natively, during evaluation and locking; outside the sandbox.
+The existing implementation based on the Git CLI had issues regarding reproducibility and performance.
+
+Most of the original fetchGit behavior has been implemented using the libgit2 library, which gives the fetcher fine grained control.
+
+Known issues:
+- The `export-subst` behavior has not been reimplemented. [Partial](https://github.com/NixOS/nix/pull/9391#issuecomment-1872503447) support for this Git feature is feasible, but it did not make the release window.