cmd/go: delete 'go doc'
It's next to useless and confusing as well. Let's make godoc better instead.

Fixes issue 4849.

R=golang-dev, dsymonds, adg, rogpeppe, rsc
CC=golang-dev
https://codereview.appspot.com/12974043
doc/go1.2.txt: three-index slices are going into 1.2

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12931044
doc/go1.2.txt: delete 'go doc'
It's nearly useless and for plain 'go doc', 'godoc .' works fine.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/13004044