doc: avoid mentioning non-existence of u flag to fmt.Printf.

doc: avoid mentioning non-existence of u flag to fmt.Printf.

It is better to document what golang does, rather than how it differs
from languages which readers may or may not know.

That the output format is based on the type is basically self-evident
if you consider this just in go terms.

Change-Id: I0223e9b4cb67cc83a9ebe4d424e6c151d7ed600f
Reviewed-on: https://go-review.googlesource.com/28393
Reviewed-by: Rob Pike <r@golang.org>