cmd/godoc: move note argument to godoc.go

Fixes the App Engine version of godoc. The other fix is to duplicate
this code inside appinit.go. I think initHandlers is the right place
to put the strings.Split call, as the notesToShow var is used by
docServer, which is what initHandlers sets up.

R=dsymonds
CC=golang-dev
https://codereview.appspot.com/7434044