Issue description

Curently with new versions of deps - build is broken.

Example of error instance:

Line 6592:
  6589	 <!-- These docs are generated via nixdoc. To add another generated
  6590	      library function file to this list, the file
  6591	      `lib-function-docs.nix` must also be updated. -->
  6592	 <xi:include href="./library/generated/strings.xml"/>
  6593	 <xi:include href="./library/generated/trivial.xml"/>
  6594	 <xi:include href="./library/generated/lists.xml"/>
  6595	 <xi:include href="./library/generated/debug.xml"/>
     error: element "xi:include" not 
    allowed here; expected the element 
    end-tag or element "bibliography", 
    "glossary", "index", "section", 
    "simplesect" or "toc"

Full log:
NixPkgsManualMakeDebug.log

This creates a blocker for contributors to the manual.
To contribute to NixPkgs manual, it is required to build the manual (link) before creating a pull request.
I want to contribute some section. By the process - I need to pass a successful build. The manual on the master branch is currently broken, building it on my setup. Errors ask me to make changes, which doc maintainers probably know what to do about.

Steps to reproduce

git pull
# currently f89f2478b89c3293d342b17efca3c6c559f52b68
cd doc
nix-shell
[nix-shell]$ make
# observe log and Error 1 result 
[nix-shell]$ make debug
# observe full log

Technical details

• system: "x86_64-linux"
• host os: Linux 4.19.16, NixOS, 19.03pre166987.bc41317e243 (Koi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.2
• channels(root): "nixos-19.03pre166987.bc41317e243, unstable-19.03pre166987.bc41317e243"
• channels(pyro): ""
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos