Rewrite comment at head of eventdns.c

Let's make it more obvious to the everyday reader that eventdns.c is
  a) Based on Libevent's evdns.c
  b) Slated for demolition
  c) Supposed to keep API-compatibility with Libevent.
  d) Not worth tweaking unless there's a bug.

src/or/eventdns.c

@@ -1,12 +1,18 @@
-/* The original version of this module was written by Adam Langley; for
- * a history of modifications, check out the subversion logs.
+/* READ THIS COMMENT BEFORE HACKING THIS FILE.
  *
- * When editing this module, try to keep it re-mergeable by Adam.  Don't
- * reformat the whitespace, add Tor dependencies, or so on.
+ * This eventdns.c copy has diverged a bit from Libevent's version, and it's
+ * no longer easy to resynchronize them.  Once Tor requires Libevent 2.0, we
+ * will just dump this file and use Libevent's evdns code.
  *
- * TODO:
- *	 - Replace all externally visible magic numbers with #defined constants.
- *	 - Write documentation for APIs of all external functions.
+ * Therefore, you probably shouldn't make any change here without making it to
+ * Libevent as well: it's not good for the implementation to diverge even
+ * more.  Also, we can't shouldn't wantonly the API here (since Libevent APIs
+ * can't change in ways that break user behavior).  Also, we shouldn't bother
+ * with cosmetic changes: the whole module is slated for demolition, so
+ * there's no point dusting the linebreaks or re-painting the parser.
+ *
+ * (We can't just drop the Libevent 2.0 evdns implementation in here instead,
+ * since it depends pretty heavily on parts of Libevent 2.0.)
  */
 
 /* Async DNS Library