docs: use a single nix-build for all the generate function docs

doc/doc-support/lib-function-docs.nix

@@ -0,0 +1,26 @@
+# Generates the documentation for library functons via nixdoc. To add
+# another library function file to this list, the include list in the
+# file `doc/functions/library.xml` must also be updated.
+
+{ pkgs ? import ./.. {}, locationsXml }:
+
+with pkgs; stdenv.mkDerivation {
+  name = "nixpkgs-lib-docs";
+  src = ./../../lib;
+
+  buildInputs = [ nixdoc ];
+  installPhase = ''
+    function docgen {
+      nixdoc -c "$1" -d "$2" -f "../lib/$1.nix"  > "$out/$1.xml"
+    }
+
+    mkdir -p $out
+    ln -s ${locationsXml} $out/locations.xml
+
+    docgen strings 'String manipulation functions'
+    docgen trivial 'Miscellaneous functions'
+    docgen lists 'List manipulation functions'
+    docgen debug 'Debugging functions'
+    docgen options 'NixOS / nixpkgs option handling'
+  '';
+}