Add note about Sec-Purpose: Prefetch

fetch.bs

@@ -5313,9 +5313,15 @@ run these steps:
    <li><p><a abstract-op lt="append the Fetch metadata headers for a request">Append the Fetch metadata headers for <var>httpRequest</var></a>.
    [[!FETCH-METADATA]]
 
-   <li><p>If <var>httpRequest</var>'s <a for=request>initiator</a> is "<code>prefetch</code>", then
-   <a for="header list">append</a> (`<code>Sec-Purpose</code>`, "<code>prefetch</code>") to
-   <var>httpRequest</var>'s <a for=request>header list</a>.
+   <li>
+    <p>If <var>httpRequest</var>'s <a for=request>initiator</a> is "<code>prefetch</code>", then
+    <a for="header list">append</a> (`<code>Sec-Purpose</code>`, "<code>prefetch</code>") to
+    <var>httpRequest</var>'s <a for=request>header list</a>.
+
+    <p class=note>This header gives servers an opportunity to handle prefetches differently from
+    regular requests. For example, the server may allow caching for a longer period for a prefetch
+    request, disallow prefetches by rejecting the request, or treat it differently for the purpose
+    of counting page visits.
 
    <li><p>If <var>httpRequest</var>'s <a for=request>header list</a>
    <a for="header list">does not contain</a> `<code>User-Agent</code>`, then user agents should