What is the documentation issue?

It says:

sizes="any" is added to icons when the extension is .svg or the image size of the file is not determined. More details in this favicon handbook

This is not true. It will add a sizes=123x123, where 123x123 is the intrinsic SVG size defined by (in my case at least) the viewBox attribute. sizes=any would be a better thing to do, since that's what SVG technically is for, but that's not what's happening at the moment.

Is there any context that might help us understand?

I've noticed this on Next.js 15.0.3, so maybe it has changed from a previous version, in which case it might also be a bug.

Does the docs page already exist? Please link to it.

https://nextjs.org/docs/app/api-reference/file-conventions/metadata/app-icons