Non ul or ol based .nav-link should require .nav-item class

Created by: ysds

The non ul or ol based navs e.g.

<nav class="nav">
  <a class="nav-link active" href="#">Active</a>
  <a class="nav-link" href="#">Link</a>
  <a class="nav-link" href="#">Link</a>
</nav>

should be require .nav-item class:

<nav class="nav">
  <a class="nav-item nav-link active" href="#">Active</a>
  <a class="nav-item nav-link" href="#">Link</a>
  <a class="nav-item nav-link" href="#">Link</a>
</nav>

Because there are many snippets without .nav-item class in the docs, so it looks like it's not necessary, but the .nav-item class is often required. As an example, the pill style does not require .nav-item, but the tab style needs it (this usage isn't documented).

demo: https://codepen.io/anon/pen/wOJWJo

And the example in the navbar nav use .nav-item with .nav-link.

This lack of consistency caused the problem of #28117 (closed).

I strongly recommend add .nav-item to all .nav-link of the non <ul> or <ol> based nav.