buildDunePackage: fix doc installation

The default directory where dune expects docs is in
`$out/doc`, but Nix installs it in `$out/share/doc`

Changed files
+4 -1
pkgs
build-support
ocaml
+4 -1
pkgs/build-support/ocaml/dune.nix
···
'';
installPhase = ''
runHook preInstall
-
dune install --prefix $out --libdir $OCAMLFIND_DESTDIR ${pname}
+
dune install --prefix $out --libdir $OCAMLFIND_DESTDIR ${pname} \
+
${if lib.versionAtLeast Dune.version "2.9"
+
then "--docdir $out/share/doc"
+
else ""}
runHook postInstall
'';