Refresh the Ocaml package examples; document useDune2 (#127520)

The current example in the manual no longer builds, mainly because
`useDune2 = true` is required, but also because the inputs have changed.
The new examples are copied verbatim from nixpkgs.

doc/languages-frameworks/ocaml.section.md

@@ -4,60 +4,83 @@ OCaml libraries should be installed in `$(out)/lib/ocaml/${ocaml.version}/site-l
 
 Given that most of the OCaml ecosystem is now built with dune, nixpkgs includes a convenience build support function called `buildDunePackage` that will build an OCaml package using dune, OCaml and findlib and any additional dependencies provided as `buildInputs` or `propagatedBuildInputs`.
 
-Here is a simple package example. It defines an (optional) attribute `minimumOCamlVersion` that will be used to throw a descriptive evaluation error if building with an older OCaml is attempted. It uses the `fetchFromGitHub` fetcher to get its source. It sets the `doCheck` (optional) attribute to `true` which means that tests will be run with `dune runtest -p angstrom` after the build (`dune build -p angstrom`) is complete. It uses `alcotest` as a build input (because it is needed to run the tests) and `bigstringaf` and `result` as propagated build inputs (thus they will also be available to libraries depending on this library). The library will be installed using the `angstrom.install` file that dune generates.
+Here is a simple package example.
+
+- It defines an (optional) attribute `minimalOCamlVersion` that will be used to
+  throw a descriptive evaluation error if building with an older OCaml is
+  attempted.
+
+- It uses the `fetchFromGitHub` fetcher to get its source.
+
+- `useDune2 = true` ensures that the latest version of Dune is used for the
+  build (this may become the default value in a future release).
+
+- It sets the optional `doCheck` attribute such that tests will be run with
+  `dune runtest -p angstrom` after the build (`dune build -p angstrom`) is
+  complete, but only if the Ocaml version is at at least `"4.05"`.
+
+- It uses the package `ocaml-syntax-shims` as a build input, `alcotest` and
+  `ppx_let` as check inputs (because they are needed to run the tests), and
+  `bigstringaf` and `result` as propagated build inputs (thus they will also be
+  available to libraries depending on this library).
+
+- The library will be installed using the `angstrom.install` file that dune
+  generates.
 
 ```nix
-{ lib
-, fetchFromGitHub
-, buildDunePackage
-, alcotest
-, result
-, bigstringaf
-}:
+{ lib,
+  fetchFromGitHub,
+  buildDunePackage,
+  ocaml,
+  ocaml-syntax-shims,
+  alcotest,
+  result,
+  bigstringaf,
+  ppx_let }:
 
 buildDunePackage rec {
   pname = "angstrom";
-  version = "0.10.0";
+  version = "0.15.0";
+  useDune2 = true;
 
-  minimumOCamlVersion = "4.03";
+  minimalOCamlVersion = "4.04";
 
   src = fetchFromGitHub {
     owner  = "inhabitedtype";
     repo   = pname;
     rev    = version;
-    sha256 = "0lh6024yf9ds0nh9i93r9m6p5psi8nvrqxl5x7jwl13zb0r9xfpw";
+    sha256 = "1hmrkdcdlkwy7rxhngf3cv3sa61cznnd9p5lmqhx20664gx2ibrh";
   };
 
-  buildInputs = [ alcotest ];
+  checkInputs = [ alcotest ppx_let ];
+  buildInputs = [ ocaml-syntax-shims ];
   propagatedBuildInputs = [ bigstringaf result ];
-  doCheck = true;
+  doCheck = lib.versionAtLeast ocaml.version "4.05";
 
-  meta = with lib; {
+  meta = {
     homepage = "https://github.com/inhabitedtype/angstrom";
     description = "OCaml parser combinators built for speed and memory efficiency";
-    license = licenses.bsd3;
-    maintainers = with maintainers; [ sternenseemann ];
+    license = lib.licenses.bsd3;
+    maintainers = with lib.maintainers; [ sternenseemann ];
   };
-}
 ```
 
 Here is a second example, this time using a source archive generated with `dune-release`. It is a good idea to use this archive when it is available as it will usually contain substituted variables such as a `%%VERSION%%` field. This library does not depend on any other OCaml library and no tests are run after building it.
 
 ```nix
-{ lib
-, fetchurl
-, buildDunePackage
-}:
+{ lib, fetchurl, buildDunePackage }:
 
 buildDunePackage rec {
   pname = "wtf8";
-  version = "1.0.1";
+  version = "1.0.2";
 
-  minimumOCamlVersion = "4.01";
+  useDune2 = true;
+
+  minimalOCamlVersion = "4.02";
 
   src = fetchurl {
-    url = "https://github.com/flowtype/ocaml-${pname}/releases/download/v${version}/${pname}-${version}.tbz";
-    sha256 = "1msg3vycd3k8qqj61sc23qks541cxpb97vrnrvrhjnqxsqnh6ygq";
+    url = "https://github.com/flowtype/ocaml-${pname}/releases/download/v${version}/${pname}-v${version}.tbz";
+    sha256 = "09ygcxxd5warkdzz17rgpidrd0pg14cy2svvnvy1hna080lzg7vp";
   };
 
   meta = with lib; {