flag: document that custom usage functions are free to call os.Exit

flag: document that custom usage functions are free to call os.Exit

Some custom usage functions call it for clarity; others rely on the default
behavior, which makes an explicit call redundant. Document that it's
safe to be explicit.

Fixes #21671.

Change-Id: I08e9f47265582821cfd35995dff0c589cd85809d
Reviewed-on: https://go-review.googlesource.com/59792
Reviewed-by: Dominik Honnef <dominik@honnef.co>
Reviewed-by: Ian Lance Taylor <iant@golang.org>