editorial: Use "content navigable" instead of "nested navigable"

The term was renamed in the HTML spec in whatwg/html#8712 so the current
reference was broken as mentioned in #1735.

While here, use ReSpec's shorthand syntax to refer to these sub concepts
directly and avoid having to specifically link to them in the index at the
end of the document.

index.html

@@ -3573,8 +3573,8 @@ <h3><dfn>Switch To Frame</dfn></h3>
       return <a>error</a> with <a>error code</a> <a>no such frame</a>.
 
      <li><p><a>Set the current browsing context</a>
-     with <var>element</var>’s <a>nested navigable</a>'s <a>active
-     browsing context</a>.
+     with <var>element</var>’s [=navigable container/content navigable=]'s
+     [=navigable/active browsing context=].
     </ol>
  </dl>
 
@@ -11059,7 +11059,6 @@ <h2>Index</h2>
    <!-- 2D context creation algorithm --> <li><dfn><a href=https://html.spec.whatwg.org/#2d-context-creation-algorithm>2D context creation algorithm</a></dfn>
    <!-- A serialization of the bitmap as a file --> <li><dfn data-lt="a serialization of the bitmap as a file"><a href=https://html.spec.whatwg.org/#a-serialisation-of-the-bitmap-as-a-file>A serialization of the bitmap as a file</a></dfn>
    <!-- API length --> <li><dfn><a href="https://html.spec.whatwg.org/#concept-fe-api-value">API value</a></dfn>
-   <!-- Active browsing context --> <li><dfn><a href=https://html.spec.whatwg.org/#nav-bc>active browsing context</a></dfn>
    <!-- Active document --> <li><dfn><a href=https://html.spec.whatwg.org/#active-document>active document</a></dfn>
    <!-- Active element --> <li><dfn>Active element</dfn> being the <a href=https://html.spec.whatwg.org/#dom-document-activeelement><code>activeElement</code></a> attribute on <a href=https://html.spec.whatwg.org/#document><code>Document</code></a>
    <!-- Associated window --> <li><dfn><a href=https://html.spec.whatwg.org/#concept-document-window>Associated window</a></dfn>
@@ -11089,7 +11088,6 @@ <h2>Index</h2>
    <!-- Mature (navigation) --> <li><dfn data-lt="matured"><a href=https://html.spec.whatwg.org/#concept-navigate-mature>Mature</a></dfn> navigation.
    <!-- Mutable --> <li><dfn><a href=https://html.spec.whatwg.org/#concept-fe-mutable>Mutable</a></dfn>
    <!-- Navigate --> <li><dfn data-lt="navigating|navigation"><a href=https://html.spec.whatwg.org/#navigate>Navigate</a></dfn>
-   <!-- Nested navigable --> <li><dfn><a href=https://html.spec.whatwg.org/#nested-navigable>nested navigable</a></dfn>
    <!-- Origin-clean --> <li><dfn><a href=https://html.spec.whatwg.org/#concept-canvas-origin-clean>Origin-clean</a></dfn>
    <!-- Overridden reload --> <li><dfn><a href="https://html.spec.whatwg.org/multipage/dom.html#an-overridden-reload">An overridden reload</a></dfn>
    <!-- Parent browsing context --> <li><dfn><a href=https://html.spec.whatwg.org/#parent-browsing-context>Parent browsing context</a></dfn>