undo CL 12822043 / 96fefaa02ae3

Fixes issue 6160.

??? original CL description
go/doc: permit a package synopsis to end with ":\n".

R=gri, r
CC=golang-dev
https://codereview.appspot.com/12822043
???

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13040043