[Documentation] Clarify that REUSE.toml does not affect reuse annotation ...

[RobertoRoos]

I would have liked a note like this, either in https://reuse.software/spec-3.2/#reusetoml or https://reuse.readthedocs.io/en/stable/man/reuse-annotate.html

I was expecting that running reuse annotate ... would take info like the copyright text from REUSE.toml, but it doesn't.
That's fine, I see there is an issue for this too: #921 . Until then I think this could be clarified in the docs.

Thanks!