[Question]shared server特性文档优化

[digoal]

Describe the problem

在shared server对比pgbouncer的文档中需要注明一下pgbouncer版本, 新版本pgbouncer支持xact模式绑定变量. https://apsaradb.github.io/PolarDB-for-PostgreSQL/zh/features/v11/performance/shared-server.html

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