doc: mentioned the "bad request" error in the "Limitations" section.

Author: agentzh

README.markdown

@@ -587,7 +587,8 @@ header_filter_by_lua* where the ngx_lua cosocket API is not available.
 because it will then be shared by all the concurrent requests handled by the same nginx
  worker process (see
 http://wiki.nginx.org/HttpLuaModule#Data_Sharing_within_an_Nginx_Worker ) and
-result in bad race conditions when concurrent requests are trying to use the same `resty.redis` instance.
+result in bad race conditions when concurrent requests are trying to use the same `resty.redis` instance
+(you would see the "bad request" or "socket busy" error to be returned from the method calls).
 You should always initiate `resty.redis` objects in function local
 variables or in the `ngx.ctx` table. These places all have their own data copies for
 each request.