sync: clarify the validity to call Map methods inside Range

This change clarifies that calling all Map methods inside the callback
of Range is allowed. For further assurance, a nested range call test
is also added.

Fixes #46399

Change-Id: I0a766a5c1470e6b573ec35df1ccd62b2e46f1561
Reviewed-on: https://go-review.googlesource.com/c/go/+/337389


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>