mention the openssl callbacks for SSL multithread

docs/examples/multithread.c

@@ -15,7 +15,15 @@
 #include <pthread.h>
 #include <curl/curl.h>
 
-/* silly list of test-URLs */
+/*
+  List of URLs to fetch.
+
+  If you intend to use a SSL-based protocol here you MUST setup the OpenSSL
+  callback functions as described here:
+
+  http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION
+
+*/
 char *urls[]= {
   "http://curl.haxx.se/",
   "ftp://cool.haxx.se/",
@@ -28,10 +36,8 @@ void *pull_one_url(void *url)
   CURL *curl;
 
   curl = curl_easy_init();
-
   curl_easy_setopt(curl, CURLOPT_URL, url);
-  curl_easy_perform(curl);
-
+  curl_easy_perform(curl); /* ignores error */
   curl_easy_cleanup(curl);
 
   return NULL;