Conform phrasing in exception of 1.4.13 (#4741)

Replaces “controlled by the user agent” with “determined by the user
agent” because that is the phrasing used by other SC.

SC 1.4.13 Content on Hover or Focus is the only SC using **controlled**
by the user agent. Five other SC phrase this as **determined** by the
user agent. Since we mean the same thing in all five places, we should
use the same phrasing in all five places. The unique phrasing in 1.4.13
implies that there is some kind of difference.

(cherry picked from commit 13c6f9e)

Author: bruce-usab

guidelines/sc/21/content-on-hover-or-focus.html

@@ -19,9 +19,9 @@ <h4>Content on Hover or Focus</h4>
 
   </dl>
 
-    	<p>Exception: The visual presentation of the additional content is controlled by the <a>user agent</a> and is not modified by the author.</p>
+    	<p>Exception: The visual presentation of the additional content is determined by the <a>user agent</a> and is not modified by the author.</p>
 
-    	<p class="note">Examples of additional content controlled by the user agent include browser tooltips created through use of the HTML <a href="https://www.w3.org/TR/html/dom.html#the-title-attribute"><code>title</code> attribute</a>.</p>
+    	<p class="note">Examples of additional content where the visual presentation is determined by the user agent include browser tooltips created through use of the HTML <a href="https://www.w3.org/TR/html/dom.html#the-title-attribute"><code>title</code> attribute</a>.</p>
      <p class="note">Custom tooltips, sub-menus, and other nonmodal popups that display on hover and focus are examples of additional content covered by this criterion.</p>
 
   </section>