In the tzdata database CEST is not recognized as a timezone name.
It is used as the abbreviated name for daylight saving time in
Central Europe.  Avoid using CEST in documentation as it suggests
that programs can parse dates that use CEST, which will typically
fail on Unix systems.

Updates #34913

Change-Id: I4b22f7d06607eb5b066812a48af58edd95498286
Reviewed-on: https://go-review.googlesource.com/c/go/+/201197
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>