go/doc: restore handling of multi-paragraph BUG comments

It was lost when the generic "Notes" support went in.

Had to change the test setup, because it precluded even
being able test multi-line comments, much less multi-paragraph
comments.

Now 'godoc sync/atomic' works correctly again.

Fixes #6135.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13427045