add a note on the rest pre processor

src/main/java/org/elasticsearch/rest/RestPreProcessor.java

@@ -40,6 +40,9 @@ public interface RestPreProcessor {
     /**
      * Process the request, returning <tt>false</tt> if no further processing should be done. Note,
      * make sure to send a response if returning <tt>false</tt>, otherwise, no response will be sent.
+     * <p/>
+     * It is recommended that the process method will not do blocking calls, or heavily cache data
+     * if a blocking call is done.
      */
     boolean process(RestRequest request, RestChannel channel);
 }