Say what the root element seam cannot do yet

RootElementContext's documentation promised that Element Call confines its
decoration to the given element, but several selectors still name body
directly and the initializer writes data-platform onto it. A non-body root
is decorated correctly and styled incorrectly, with no error to show for
it, so the documentation should say so until the stylesheets are scoped.

src/RootElementContext.ts

@@ -12,9 +12,17 @@ import { createContext, use } from "react";
  *
  * Element Call decorates this element with the theme, layout and background
  * attributes its stylesheets key off, and portals its modals into it. When
- * Element Call owns the page this is simply the document body; when it is
+ * Element Call owns the page this is simply the document body; the intent is
- * embedded in a host application it is the container the host mounted it into,
+ * that when embedded in a host application it becomes the container the host
- * so that Element Call does not reach outside its own subtree.
+ * mounted it into, so that Element Call does not reach outside its own subtree.
+ *
+ * That intent is not yet achievable: several selectors still name `body`
+ * directly — `body[data-background="gradient"]` and `body[data-platform=…]` in
+ * `index.css`, and `body[data-platform="ios"]` in `AppBar.module.css` and
+ * `Modal.module.css` — and `Initializer.initBeforeReact` writes
+ * `data-platform` straight onto the body. So anything other than the body will
+ * be decorated correctly and styled incorrectly, silently. Until those are
+ * scoped, treat this as preparation rather than a working seam.
  */
 const RootElementContext = createContext<HTMLElement | null>(null);