clarify a comment in the tor-exit-notice text

(a relay operator in #tor-relays just now was confused and thought
that dirportfrontpage could serve multiple files, like a real webserver.)

contrib/operator-tools/tor-exit-notice.html

@@ -38,8 +38,11 @@ router IP should be generating no other traffic, unless it has been
 compromised.</p>
 
 
-<!-- FIXME: you should probably grab your own copy of how_tor_works_thumb.png
-     and serve it locally -->
+<!-- FIXME: you should consider grabing your own copy of
+     how_tor_works_thumb.png and serving it locally. But note that if
+     you're serving this file with Tor's DirPortFrontPage option, it
+     can only serve a single file, so you would need to put this image
+     up on your own webserver somewhere if you want a local copy. -->
 
 <p style="text-align:center">
 <a href="https://www.torproject.org/about/overview">