A bit of confusion terminolgoy about `TiKV` `TiKV store` `TiKV peer` `peer: replica of Region` in tidb-scheduling.md

1. When a new TiKV store is added, data can be rebalanced to it;
2. The cluster can perform automatic disaster recovery from TiKV peers' failure.
3. Note that here "position" is different from "machine". Generally PD can only ensure that replicas of a Region are not at a same peer to avoid that the peer's failure causes more than one replicas to become lost. However in production, you might have the following requirements:

Region is the minimal piece of data storage in TiKV, each representing a range of data (96 MiB by default). Each Region has three replicas by default. A replica of a Region is called a peer. Multiple peers of the same Region replicate data via the Raft consensus algorithm, so peers are also members of a Raft instance. TiKV uses Multi-Raft to manage data. That is, for each Region, there is a corresponding, isolated Raft group.

Open tidb-scheduling.md and review the uses of “TiKV,” “TiKV store,” “TiKV peer,” and “peer” in the reported passages. Clarify the terminology and the distinction between a peer, a replica, a store, and a machine; the issue is done when these terms are consistent and unambiguous.