Rollup merge of #143941 - folkertdev:cfg-select-docs, r=traviscross

update `cfg_select!` documentation

tracking issue: rust-lang/rust#115585

After rust-lang/rust#143461, and with an eye on a soon(ish) stabilization, I think the docs need some work.

The existing text read more like a motivation for the feature existing to me, so I've tried to now be a bit more descriptive. Still, suggestions are very welcome.

I also added a test for an empty `select! {}` because it's just the sort of thing that might break.

r? ``@traviscross``

Author: samueltardieu