spec: adjust for-range prose to include iterator functions where missing
Also add a missing table entry to the language versions section in the
appendix.
Fixes #67977.
Change-Id: I1f98abbbcc34a4ff31f390752635435eaf6120b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/592595
TryBot-Bypass: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
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