Fix inaccuracies in replication explanation

[SiefSeif]

Changelog

Synchronous replication means the leader waits for one or more replicas to acknowledge a write before confirming success to the client. This ensures strong consistency — if a write succeeds, it exists on all replicas — but increases write latency.

Asynchronous replication means the leader acknowledges a write immediately, then propagates it to replicas later. This provides better performance and availability, but replicas can temporarily lag (eventual consistency).

In databases, asynchronous replication usually happens continuously in near real-time; in backup or storage systems, it may happen on a schedule for cost reasons.

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