Warn that dependencies using backpack are broken when installed from …

…hackage (#6264)

* Warn the dependencies using backpack are broken when installed from hackage

* Reword

* Update nix-local-build.rst

Make the hyperlink to issue 6005 a little better.

Cabal/doc/nix-local-build.rst

@@ -653,6 +653,14 @@ byte) that must be satisfied for it to function correctly in the larger v2-build
 ``autogen-modules`` is able to replace uses of the hooks to add generated modules, along with
 the custom publishing of Haddock documentation to Hackage.
 
+.. warning::
+
+  Packages that use Backpack will stop working if uploaded to
+  Hackage, due to `issue #6005 <https://github.com/haskell/cabal/issues/6005>`_.
+  While this is happening, we recommend not uploading these packages
+  to Hackage (and instead referencing the package directly
+  as a ``source-repository-package``).
+
 Configuring builds with cabal.project
 =====================================