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

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang