Better mbedtls debug

docs/README.md

@@ -668,7 +668,12 @@ void mg_mgr_wakeup(struct mg_connection *pipe, const void *buf, size_len len);
 
 Wake up an event manager that sleeps in `mg_mgr_poll()` call. This function
 must be called from a separate task/thread. A calling thread can pass
-some specific data to the IO thread via `buf`, `len`. Parameters:
+some specific data to the IO thread via `buf`, `len`. There is a limitation
+on the data size that can be sent: first, it is `MG_IO_MAX` build constant,
+and second, it is a maximum UDP datagram size, which is 64KiB. If you need
+to send a large data to the Mongoose thread, `malloc()` the data and send
+a pointer to it, not the data itself. The receiving event handler can receive
+a pointer, send a response, and call `free()`. Parameters:
 
 Parameters:
 - `pipe` - a special connection created by the `mg_mkpipe()` call