lib: Discourage use of extend

It creates interoperability issues that can not be reconciled by
`lib` or maintainers of projects that use the Nixpkgs library.
Occasionally, end users may be able to solve the problems they run
into, but most are not prepared to deal with this set of problems,
nor should they be.

Typical conflict:
- User wants to propagate their own lib, because it has some function
  they like to use throughout their projects
- Project maintainer requires the project's lib to be used

No sane language uses a single namespace for combining all the things.
(Arguably not even C with its extensive use of prefixing)

Meanwhile, in Nix, all symbols are first class variables. We don't even
have the concept of a global top-level namespace to pour everything into.
With `lib` you can try to approximate that, I get the appeal of its
apparent simplicity, but since `lib` can't be global, we just don't even
get that apparent simplicity.

I apologize for not offering concrete solutions to this in the text.
The manuals are limited to reference documentation.
Alternatives - of which we have multiple - are best provided in
task-oriented documentation, e.g. nix.dev.

flake.nix

@@ -24,6 +24,8 @@
 
         - `lib.nixos` for other NixOS-provided functionality, such as [`runTest`](https://nixos.org/manual/nixos/unstable/#sec-call-nixos-test-outside-nixos)
       */
+      # DON'T USE lib.extend TO ADD NEW FUNCTIONALITY.
+      # THIS WAS A MISTAKE. See the warning in lib/default.nix.
       lib = lib.extend (final: prev: {
 
         /**