fix incorrect message for set_var() (#19451)

[ti-chi-bot]

This is an automated cherry-pick of #19451

What is changed, added or deleted? (Required)

Fix incorrect messages for set_var()

• tidb_read_consistency is supported conditionally
• tidb_read_staleness is not support

Translated from pingcap/docs-cn#19119

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• v8.5 (TiDB 8.5 versions)
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• v5.3 (TiDB 5.3 versions)

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

• This PR is translated from: fix incorrect message for set_var() docs-cn#19119
• 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

[lilin90]

/approve

[ti-chi-bot]

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