Fix top-level active link (#1015)

* Fix top-level active link

Fixes #1014

1.  Reverse the order of `page.parent == node.title or page.grand_parent == node.title`. This makes no difference.
2.  Replace `page.parent == node.title` by `page.parent == node.title  and page.grand_parent == nil`. The condition is evaluated first because it is rightmost.
    We have `node in first_level_pages`.
    The old condition holds not only when `page` is a child of `node`,
    but also when `page` is a grandchild of a *different* top-level page and its parent happens to have the same title as `node`.
    The new condition never holds for a grandchild.

This change has been tested locally: in v0.4.0.rc3, when the 3rd-level page `G` was selected, the link to the top-level page `F` was active, and the link to its child `G` was shown; after making the change, it is no longer active, so the link to its child `G` is not shown.

* Update nav.html

Add a comment to clarify just when top-level nodes are active.