mailbox: Clarify multi-thread is not supported in blocking mode

Unlike in non-blocking mode, multi-thread has not been supported in
blocking mode. This commit is to prevent clients from having wrong
assumption by explicitly specifying this fact to the API doc.

Signed-off-by: Joonwon Kang <joonwonkang@google.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

drivers/mailbox/mailbox.c

@@ -258,6 +258,10 @@ EXPORT_SYMBOL_GPL(mbox_chan_tx_slots_available);
  * over the chan, i.e, tx_done() is made.
  * This function could be called from atomic context as it simply
  * queues the data and returns a token against the request.
+ *  In blocking mode, it is caller's responsibility to serialize threads'
+ * access to a channel if multi-threads are to send messages through the
+ * same channel, i.e. caller should not call this function until any
+ * previous call returns.
  *
  * Return: Non-negative integer for successful submission (non-blocking mode)
  *	or transmission over chan (blocking mode).