gofmt: explain why lower bounds aren't automatically simplified
gofmt: explain why lower bounds aren't automatically simplified

Full credit goes to gri and rsc for their explanations.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/14229043

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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