cmd/godoc: add support for display Notes parsed by pkg/go/doc
pkg/go/doc: move BUG notes from Package.Bugs to the general Package.Notes field.
Removing .Bugs would break existing code so it's left in for now.

R=gri, gri, gary.burd, dsymonds, rsc, kevlar
CC=golang-dev
https://codereview.appspot.com/7341053

Committer: Robert Griesemer <gri@golang.org>