cmd/godoc: show the BUG section

This will match the default behavior when running locally.

In fact, our http://blog.golang.org/godoc-documenting-go-code explicitly
documents this feature (see also golang.org/cl/1953).

Change-Id: I581b17b60229ce70900cb51d548d1e2a34df41ba
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/2116
Reviewed-by: Andrew Gerrand <adg@golang.org>