Hadrian does not rebuild _build/doc/html/users_guide/flags.html

Summary

Hadrian does not rebuild _build/doc/html/users_guide/flags.html

Steps to reproduce

1. Build HTML docs (e.g. with hadrian/build -j --flavour=Quick --freeze1 docs --docs=no-sphinx-pdfs. The Quick flavor might be incidental, it's just what I use during development)
2. Change docs/users_guide/using-warnings.rst, e.g. add a new warning flag or modify one of the :shortdesc: fields.
3. Rerun the command to build HTML docs.
4. There is no change in flags.html

Expected behavior

I expected the change to be reflected in flags.html.