content: Remove outdated comment about "package documentation"

godoc was changed a while back to look for command documentation in
"package main" rather than "package documentation" to be consistent
with library package conventions:
https://code.google.com/p/go/source/detail?r=3633a89bb56d

R=adg
CC=golang-dev
https://golang.org/cl/15450044
1 file changed