Clarify manual scroll restoration mode

Scroll restoration mode does not affect nested iframes but all other
scrollable regions. Fixes #349.

Also add a note to clarify that manual scroll restoration does not
imply position (0,0). Fixes #350.

PR #406

source

@@ -82659,7 +82659,9 @@ State: <OUTPUT NAME=I>1</OUTPUT> <INPUT VALUE="Increment" TYPE=BUTTON O
 
    <li><p>If <var>scrollRestoration</var> is "<code
    data-x="dom-ScrollRestoration-manual">manual</code>" the user agent should not restore the scroll
-   position for the document, otherwise, it may do so.</p></li>
+   position for the <code>Document</code> or any of its scrollable regions, with the exception of
+   any nested browsing contexts whose scroll restoration is controlled by their own history entry's
+   <span>scroll restoration mode</span>, otherwise, it may do so.</p></li>
 
    <li><p>Optionally, update other aspects of the document and its rendering, for instance values of
    form fields, that the user agent had previously recorded.</p></li>
@@ -82675,6 +82677,11 @@ State: &lt;OUTPUT NAME=I>1&lt;/OUTPUT> &lt;INPUT VALUE="Increment" TYPE=BUTTON O
   data-x="attr-input-type-search">Search</span> state, if the persisted state includes the
   directionality of user input in such controls.</p>
 
+  <p class="note">Not restoring the scroll position by user agent does not imply that the scroll
+  position will be left at any particular value (e.g., (0,0)). The actual scroll position depends on
+  the navigation type and the user agent's particular caching strategy. So web applications should 
+  not assume any particular scroll position but rather should set it to what they want it to be.</p>
+
   <h5>The <code>PopStateEvent</code> interface</h5>
 
   <pre class="idl">[Constructor(DOMString type, optional <span>PopStateEventInit</span> eventInitDict), Exposed=(Window,Worker)]