go/doc: don't treat _ consts as exported

go/doc: don't treat _ consts as exported

golang.org/cl/144110044 made _ consts treated
as exported as a small, safe fix for #5397.
It also introduced issue #9615.

golang.org/cl/2091 then fixed the underlying issue,
which was missing type information when the type
was specified only for _.

This cl reverts the original fix.

Fixes #9615.

Change-Id: I4815ad8292bb5bec18beb8c131b48949d9af8876
Reviewed-on: https://go-review.googlesource.com/3832
Reviewed-by: Robert Griesemer <gri@golang.org>