.nav-header class missing

[bretterer]

In a drodown, the two classes are missing:

<li class="nav-header"></li>
<li class="divider"></li>

These were around until bootstrap 3.0 and there is no info on a class name change in the docs.

[cvrebert]

@bretterer .divider still exists; it's used many times in the Components docs examples.

[cvrebert]

.nav-header was part of .nav-list, so this is a duplicate of #9139.