Use proper element name for DocBook manpages.

The proper element name is "refentry", not "manpage".

This fixes #1134.
One very slight optimization (since we're obsessed with optimization in the converters)

```
if (root_tag_name = node.doctype) == 'manpage'
  root_tag_name = 'refentry'
end
```

This avoids resolving node.doctype twice. It probably has negligible impact, but I like to keep the pattern of optimization consistent. Would you be willing to update?

Btw, thanks for the tests!