Need to fix the document, or evict the global variables cache immediately

[xhebox]

Bug Report

refer to #14531 and #22806. While the behavior of this bug is expected, the documentation is problematic.

The description of this behavior is added by pingcap/docs-cn#2208 and pingcap/docs#1797, which describes that:

A single TiDB instance proactively gets the `GLOBAL` information and forms `gvc` (global variables cache) every 2 seconds.

But the document is modified to the misleading one in pingcap/docs-cn#4001 and pingcap/docs#3152.

Changes are also propagated to other TiDB servers every 2 seconds

All TiDB servers need to wait for changes to propagate, including the one you sent SET GLOBAL xxxx.

We either need to fix the document, or investigate if we can evict the GVC for the tidb-server you send SET GLOBAL xxxx immediately. I prefer the latter if it is possible to implement.

[xhebox]

/label sig/infra

[morgo]

This has been updated in the docs.

[ti-srebot]

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