fmt: document width and flags a bit more
fmt: document width and flags a bit more

Fixes issue 4581.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7000052

firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old