Update documentation for -Werror in Hadrian

I've decided it's time to modernize the way I work in Haskell/GHC, including using VSCode, HLS, and Hadrian.

I like to use the devel2 flavour, with -Werror enabled. I see in https://gitlab.haskell.org/ghc/ghc/-/blob/master/hadrian/doc/user-settings.md#enabling-werror that there is a werror function in Hadrian to enable -Werror. However, I am left with a number of questions. Instead of emailing these, I'm filing this ticket, in the hopes that the answer can be put in the official documentation.

The documentation suggests devel2WerrorFlavour = werror (developmentFlavour Stage2). But then what? This will declare a new Flavour. Presumably I have to add this to userFlavours? What is the new flavour's name? That is, how would I select it on the command line? Is the new flavour identical to devel2? If that's the case, why developmentFlavour Stage2 instead of devel2? I'm worried these could somehow get out of sync.

Also, is there a way of setting -Werror on the command line? Command-line access seems possible for many other features, but there's nothing in the docs about -Werror.

Thanks!