It was inconsistent.
Also test these better.
Also document the default format for types.
This wasn't written down.
Fixes#8470.
LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/154870043
Previous behavior was undocumented and inconsistent. Now it is documented
and consistent and measures the input size, since that makes more sense
when talking about %q and %x. For %s the change has no effect.
Fixes#8151.
LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/144540044