doc/lib-functions: remove warnings

Building the nixpkgs manual currently triggers a bunch of deprecation
warnings, because every attribute in `lib` is evaluated to see if it's
an attrset to generate locations for.

Instead, share the lib subsets to include in the documentation
between `lib-function-docs` and `lib-function-locations` so they can
coordinate.

Also generate the list of sections instead of duplicating it in
`library.xml`.

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

@@ -1,4 +1,4 @@
-{ pkgs ? (import ./.. { }), nixpkgs ? { }}:
+{ pkgs, nixpkgs ? { }, libsets }:
 let
   revision = pkgs.lib.trivial.revisionWithDefault (nixpkgs.revision or "master");
 
@@ -16,9 +16,7 @@ let
         subsetname = subsetname;
         functions = libDefPos toplib.${subsetname};
       })
-      (builtins.filter
-        (name: builtins.isAttrs toplib.${name})
-        (builtins.attrNames toplib));
+      (builtins.attrNames libsets);
 
   nixpkgsLib = pkgs.lib;