Editorial: add mention that RegExp is constructible (tc39#3055)

spec.html

@@ -37165,7 +37165,8 @@ <h1>The RegExp Constructor</h1>
       <ul>
         <li>is <dfn>%RegExp%</dfn>.</li>
         <li>is the initial value of the *"RegExp"* property of the global object.</li>
-        <li>when called as a function, returns either a new RegExp object, or the argument itself if the only argument is a RegExp object.</li>
+        <li>creates and initializes a new RegExp object when called as a constructor.</li>
+        <li>when called as a function rather than as a constructor, returns either a new RegExp object, or the argument itself if the only argument is a RegExp object.</li>
         <li>may be used as the value of an `extends` clause of a class definition. Subclass constructors that intend to inherit the specified RegExp behaviour must include a `super` call to the RegExp constructor to create and initialize subclass instances with the necessary internal slots.</li>
       </ul>