go.tools/cmd/godex: update documentation
go.tools/cmd/godex: update documentation

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/83580046

karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools