maps: clarify iteration order and insertion behavior
Fixes #67537.
Change-Id: Ic567f7d19d621a17d2a00aba5b9f927001195ea9
GitHub-Last-Rev:
9eec790ab6f1697f6328220e007fee8661f62c5c
GitHub-Pull-Request: golang/go#67539
Reviewed-on: https://go-review.googlesource.com/c/go/+/587015
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>