Be more reassuring about cross-platform semantics

(My summary/analysis of this Discord discussion.)

After we've worked so hard to preserve cross-platform semantics, at least I have forgotten that apparently this is surprising for users. When the expectation is that Scala.js support is second-class, users may doubt whether certain things work or various advice applies and look to docs/support to answer their questions.

So, we probably need to be more reassuring and upfront about the fact that everything pretty much works the same. I think for the most part, that unless otherwise noted, basically all the docs apply to both JVM and JS.

I have a bit of this in my feral README.
https://github.com/typelevel/feral#why-go-feral