docs: Clarify the monotonic time-in-state documentation

[tanderson-ld]

Follow-up to #459, documentation only.

• Explains on fallback_condition / recovery_condition why the caller supplies the elapsed time (the monotonic baseline is private to the status provider).
• Tightens the status_and_seconds_in_state doc comment to three sentences: what it returns, why one lock acquisition, and which value to use for duration decisions.

Matches the wording used in the equivalent python-server-sdk change.

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Note

Overview
Documentation-only follow-up clarifying how monotonic time-in-state is documented for FDv2 failover/recovery.

The StatusProviderV2#status_and_seconds_in_state comment is tightened to three sentences: what the method returns, why a single lock keeps status and duration consistent, and that callers should use the returned monotonic seconds (not state_since math) for duration decisions.

fallback_condition and recovery_condition in FDv2 now document why seconds_in_state is passed in by the caller—the monotonic baseline lives inside the status provider—and point readers to status_and_seconds_in_state. Wording aligns with the equivalent python-server-sdk change.

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