Clarify request's mode "cors" value

fetch.bs

@@ -1390,8 +1390,10 @@ to not have to set <a for=/>request</a>'s <a for=request>referrer</a>.
   <a>network error</a> if the request is not made to a same-origin URL.
 
   <dt>"<code>cors</code>"
-  <dd>Makes the request a <a>CORS request</a>. Fetch will return a <a>network error</a> if the
-  requested resource does not understand the <a>CORS protocol</a>.
+  <dd>For requests whose <a for=request>response tainting</a> gets set to "<code>cors</code>", makes
+  the request a <a>CORS request</a> — in which case, fetch will return a <a>network error</a> if the
+  requested resource does not understand the <a>CORS protocol</a>, or if the requested resource is
+  one that intentionally does not participate in the <a>CORS protocol</a>.
 
   <dt>"<code>no-cors</code>"
   <dd>Restricts requests to using <a>CORS-safelisted methods</a> and