README: clarify tx deadline behavior for canardTxPoll (#255)

This fixes the ambiguity reported in #159 comments where `tx_deadline_usec = 0` looked like an unlimited deadline in examples.

cc @pavel-kirienko @blutack

---------

Co-authored-by: laktoosivaba <rooh1622@gmail.com>

Author: MAMONTBOT

README.md

@@ -116,7 +116,7 @@ const struct CanardTransferMetadata transfer_metadata = {
 ++my_message_transfer_id;  // The transfer-ID shall be incremented after every transmission on this subject.
 int32_t result = canardTxPush(&queue,               // Call this once per redundant CAN interface (queue).
                               &canard,
-                              tx_deadline_usec,     // Zero if transmission deadline is not limited.
+                              tx_deadline_usec,     // Transmission deadline (absolute monotonic time in usec).
                               &transfer_metadata,
                               47,                   // Size of the message payload (see Nunavut transpiler).
                               "\x2D\x00" "Sancho, it strikes me thou art in great fear.",

libcanard/canard.h

@@ -519,7 +519,7 @@ struct CanardTxQueue canardTxInit(const size_t                      capacity,
 /// Therefore, normally, the timestamp value should be in the future.
 /// The library compares (now>deadline) to determine which frames timed out, and so could
 /// be dropped (incrementing frames_expired, unless NULL).
-/// If this timeout behavior is not needed, the timestamp value can be set to zero.
+/// If this timeout behavior is not needed, pass now_usec=0 to canardTxPush()/canardTxPoll().
 ///
 /// The described above automatic dropping of timed-out frames was added in the v4 of the library as an optional
 /// feature. It is applied only to the frames that are already in the TX queue (not the new ones that are being pushed