Make the expressions of GlobalStats consistent

[lilin90]

What is changed, added or deleted? (Required)

This PR makes the expressions of GlobalStats consistent and easier to understand based on this comment pingcap/docs-cn#17827 (comment) and communication with winoros. It's a follow-up to pingcap/docs-cn#17827.

Specifically, this PR changes GlobalStats to 分区表的全局统计信息, or just 全局统计信息 when the context is clear to avoid redundancy.

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What is the related PR or file link(s)?

• This PR is translated from: Make the expressions of GlobalStats consistent docs-cn#18524
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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