Add more context to the new P/R for 4154

xml/issue4154.xml

@@ -71,12 +71,20 @@ This wording is relative to <paper num="N4988"/>.
 <ol>
 <li><p>Modify <sref ref="[futures.task.members]"/> as indicated:</p>
 <blockquote>
+<pre><code>template&lt;class F&gt;
+  explicit packaged_task(F&amp;&amp; f);</code></pre>
+<p>
+-2- <i>Constraints</i>:
+<code>remove_cvref_t&lt;F&gt;</code> is not the same type as
+<code>packaged_task&lt;R(ArgTypes...)&gt;</code>.
+</p>
 <p>
 -3- <i>Mandates</i>:
 <code>is_invocable_r_v&lt;R, <del>F</del><ins>decay_t&lt;F&gt;</ins>&amp;, ArgTypes...&gt;</code>
 is `true`.
 </p>
-<p><del>-4- <i>Preconditions</i>:
+<p>
+<del>-4- <i>Preconditions</i>:
 Invoking a copy of `f` behaves the same as invoking `f`.
 </del>
 </p>