undo CL 12822043 / 96fefaa02ae3

Fixes #6160.

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

R=gri, r
CC=golang-dev
https://golang.org/cl/12822043
»»»

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13040043