README: clarify tx deadline behavior for canardTxPoll

[MAMONTBOT]

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

Changes

• Replace the misleading inline comment near canardTxPush():
  • from: "Zero if transmission deadline is not limited"
  • to: "Transmission deadline (absolute monotonic time in usec)"
• Add an explicit note near canardTxPoll() usage:
  • with non-zero now_usec, zero deadline means immediate expiration;
  • use a future deadline, or pass now_usec=0 to disable timeout-based dropping.

cc @pavel-kirienko @blutack

[pavel-kirienko]

The header docs still have passages like "If this timeout behavior is not needed, the timestamp value can be set to zero." which should be also updated throughout.

[pavel-kirienko]

Suggested change

	⚠️ When using `canardTxPoll()` with non-zero `now_usec`, a zero deadline means the frame is already expired.
	Use a future deadline value, or pass `now_usec=0` to disable timeout-based dropping.

causes doc fatigue

[MAMONTBOT]

Thanks, good catch — fixed in the header docs as well.

Updated in libcanard/canard.h:

• replaced the misleading guidance about setting deadline to zero;
• clarified that timeout disabling is done via now_usec=0 in canardTxPush()/canardTxPoll();
• clarified that zero deadline is not treated as unlimited when timeout dropping is enabled.

Mamont 🦣 (Pavel’s assistant)

[pavel-kirienko]

Suggested change

	/// If this timeout behavior is not needed, pass now_usec=0 to canardTxPush()/canardTxPoll().
	/// A zero deadline is not treated as "unlimited" when timeout dropping is enabled.

[MAMONTBOT]

Applied your suggestion from the review discussion (discussion_r2834067945) and removed that README warning block.

Mamont 🦣 (Pavel’s assistant)

[MAMONTBOT]

Applied. I removed the extra blank line in README and pushed the cleanup.

I’ll keep applying all your suggestions and avoid cosmetic/noise edits.

Mamont 🦣 (Pavel’s assistant)

[MAMONTBOT]

Applied the remaining edit suggestion (discussion_r2834119111) and removed the extra/silly formatting churn.

Current status:

• all your suggestions in this PR have been applied;
• CI is running; I am monitoring until it is fully green (or clearly pre-existing/unrelated if any check fails).

Mamont 🦣 (Pavel’s assistant)