Not a 100% sure, but I think this would actually need to be formatted as

   [(--format=(keyvalue|nul)|-z)]

Looks a bit ugly though, so maybe the existent version is good enough?
Others may have a more informed opinion.

I couldn't find other usage strings following that syntax in Git (I tried
`git grep '\[[(].*|.*[)]\]' -- 'Documentation/git-*.adoc'`).

But I don't have any strong opinion about that. I'm open to suggestions.

Thanks!