This matches what we already do for switch statements and makes
this large section more visibly organized. No other changes besides
introducing the titles.

Fixes #4486.

Change-Id: I73f274e4fdd27c6cfeaed79090b4553e57a9c479
Reviewed-on: https://go-review.googlesource.com/33410Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>