spec: move rule for type parameters up in section of range clause

Make the rule for range expressions of type parameter type more
visible by moving it up (it was easily missed before as it was
at the end of the example section).

Add a corresponding example to the examples.

Per suggestion from @adonovan.

Change-Id: I8527e69ecc7d0110554ba00f4c30829f7d260d41
Reviewed-on: https://go-review.googlesource.com/c/go/+/764921
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>