Liuxiaozhen patch 1

[Liuxiaozhen12]

What is changed, added or deleted? (Required)

This is the docs for pingcap/tidb#24359 which has not merged yet. It more accurately reflects the future-state of sysvars.

There is another PR for GA releases which describes how it previously worked better: #5562

It needs a "followup" to insert the version the change is effective from.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

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

• This PR is translated from: Add docs for sysvar cache docs#5563
• 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

[ti-chi-bot]

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[Liuxiaozhen12]

Another PR is opened for this: #6464