CASSANDRA-20800: Updated tombstone note to clarify behavior with TTL and gc_grace_seconds

[bschoening]

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The Cassandra Jira

[Copilot]

Pull Request Overview

This PR updates documentation for Cassandra tombstones to clarify the behavior when TTL (Time-To-Live) values interact with gc_grace_seconds. The change replaces a brief mention of TTL creating tombstones with a more detailed explanation of how expired data behaves during compaction when gc_grace_seconds exceeds the original TTL.

• Updated the TTL behavior note to explain the expired flag mechanism
• Clarified the relationship between TTL expiration and gc_grace_seconds
• Added explanation of when data acts as a tombstone during compaction

[Copilot]

The documentation should clarify what happens in the more common case when TTL is greater than gc_grace_seconds, or when they are equal, to provide a complete picture of TTL behavior.

Suggested change

	This situation arises only when gc_grace_seconds exceeds the original TTL.
	- If `gc_grace_seconds` exceeds the original TTL, the expired data (tombstone) will be retained until `gc_grace_seconds` elapses, ensuring that deleted data is not resurrected due to late replica synchronization.
	- If the TTL is greater than `gc_grace_seconds`, or if they are equal, the tombstone may be eligible for removal as soon as `gc_grace_seconds` has elapsed, which could result in the data being permanently deleted before all replicas have had a chance to synchronize. This increases the risk of deleted data reappearing (zombies) if a replica was down during the grace period.
	For safety, it is generally recommended to set `gc_grace_seconds` to a value greater than or equal to the maximum TTL used in the table, to avoid the risk of zombie data.