nixos: doc: increase maxdepth for xsltproc

See https://github.com/NixOS/nixpkgs/issues/37903#issuecomment-376618117
for details. With the previous patch and some custom modules included in
`configuration.nix` the above bug is very easy to trigger.

This is a simplest workaround I have. A proper solution would look like
https://github.com/NixOS/nixpkgs/issues/37903#issuecomment-376980838.

nixos/doc/manual/default.nix

@@ -327,6 +327,7 @@ in rec {
       # Generate manpages.
       mkdir -p $out/share/man
       xsltproc --nonet \
+        --maxdepth 6000 \
         --param man.output.in.separate.dir 1 \
         --param man.output.base.dir "'$out/share/man/'" \
         --param man.endnotes.are.numbered 0 \