Make it explicit that user agents can ignore replaceState/pushState

Fixes whatwg#982. The "a user agent could ignore calls" is moved from the
non-normative implementation notes to the normative algorithm, and the
implementation notes are updated with a pointer to this.

source

@@ -79457,6 +79457,10 @@ interface <dfn>History</dfn> {
    <li><p>If this <code>History</code> object is associated with a <code>Document</code> that is
    not <span>fully active</span>, throw a <code>SecurityError</code> exception.</p></li>
 
+   <li><p>Optionally, abort these steps. (For example, the user agent might disallow calls to these
+   methods that are invoked on a timer, or from event listeners that are not triggered in response
+   to a clear user action, or that are invoked in rapid succession.)</p></li>
+
    <li><p>Let <var>targetRealm</var> be this <code>History</code> object's <span>relevant settings
    object</span>'s <span data-x="environment settings object's realm">Realm</span>.</p></li>
 
@@ -79727,10 +79731,10 @@ State: &lt;OUTPUT NAME=I>1&lt;/OUTPUT> &lt;INPUT VALUE="Increment" TYPE=BUTTON O
   two "back" commands, one that goes back to the previous state, and one that jumps straight back to
   the previous page.</p>
 
-  <p>In addition, a user agent could ignore calls to <code
-  data-x="dom-history-pushState">pushState()</code> that are invoked on a timer, or from event
-  listeners that are not triggered in response to a clear user action, or that are invoked in rapid
-  succession.</p>
+  <p>For both <code data-x="dom-history-pushState">pushState()</code> and <code
+  data-x="dom-history-replaceState">replaceState()</code>, user agents are encouraged to prevent
+  abuse of these APIs via too-frequent calls or over-large state objects. As detailed above, the
+  algorithm explicitly allows user agents to ignore any such calls when appropriate.</p>
 <!--REMOVE-TOPIC:Security-->
 
   </div>