go/types, types2: report error for floating-point iteration variable

go/types, types2: report error for floating-point iteration variable

While at it, slightly improve documentation and code.
Also, add additional test cases for #66561.

Updates #66561.
Fixes #67027.

Change-Id: I682b0e9227e065d6bbd199871c2e1ecff13edc66
Reviewed-on: https://go-review.googlesource.com/c/go/+/580937
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>