The existing docs only mention that it is possible to change the output
destination of PrintDefaults from the default of standard error, but fail to
mention how to actually do so. This change fixes this lack by directing users
to CommandLine.SetOutput.

Fixes #15024

Change-Id: Ieaa7edbebd23d4ea6fa7e53d97a87143d590bdb3
Reviewed-on: https://go-review.googlesource.com/c/145203Reviewed-by: Rob Pike <r@golang.org>