testing: update description of Setenv

testing: update description of Setenv

Add the description of Setenv that it cannot use if the test have
parallel ancestors.

Fixes #55128

Change-Id: Ia5a1deaa1a3116d1ebb439600a7d316c7d155412
Reviewed-on: https://go-review.googlesource.com/c/go/+/434115
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>