flag: improve docs for PrintDefaults; clarify how to change output destination
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/145203 Reviewed-by: Rob Pike <r@golang.org>