[dev.regabi] cmd/compile: expand documentation for Func.Closure{Vars,Enter}

I keep getting these confused and having to look at how the code
actually uses them.

Change-Id: I86baf22b76e7dddada6830df0fac241092f716bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/280993


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>