Clarify "manual" redirect mode

Closes #763.

fetch.bs

@@ -1519,8 +1519,9 @@ Unless stated otherwise, it is "<code>follow</code>".
   <dd>Return a <a>network error</a> when a request is met with a redirect.
 
   <dt>"<code>manual</code>"
-  <dd>Retrieves an <a>opaque-redirect filtered response</a> when a request is met with a redirect so
-  that the redirect can be followed manually.
+  <dd>Retrieves an <a>opaque-redirect filtered response</a> when a request is met with a redirect to
+  allow a service worker to replay the redirect offline. The response is otherwise indistinguishable
+  from a <a>network error</a> to not violate <a>atomic HTTP redirect handling</a>.
  </dl>
 </div>
 
@@ -4079,8 +4080,8 @@ optional <i>CORS-preflight flag</i>, run these steps:
   <i>recursive flag</i> set if <var>request</var>'s <a for=request>redirect mode</a> is not
   "<code>manual</code>".
 
-  <p class=note>It can only be "<code>manual</code>" when invoked directly from HTML's navigate
-  algorithm.
+  <p class=note>It can only be "<code>manual</code>" here when this algorithm is invoked directly
+  from HTML's navigate algorithm.
 
   <p class="note no-backref">This has to invoke <a for=main>main fetch</a> to
   get <a for=request>response tainting</a> correct.