docs: 修正不可重复读和幻读概念描述

[LunaSpica]

调整不可重复读和幻读的定义描述，使其更准确区分两者。

• 不可重复读强调同一条记录在事务期间被其他事务修改或删除，导致再次读取时内容或存在性发生变化。
• 幻读强调同一范围条件查询多次执行时，返回的记录集合发生变化，例如新增或删除符合条件的记录。

优化描述，避免将记录级变化与范围查询结果变化混淆。

参考：
https://www.postgresql.org/docs/current/transaction-iso.html

nonrepeatable read
A transaction re-reads data it has previously read and finds that data has been modified by another transaction (that committed since the initial read).

phantom read
A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed due to another recently-committed transaction.