Add note for setting TiDB configuration parameter during cluster upgr…

[Benjamin2037]

…ade.

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• [x] v1.5 (TiDB Operator 1.5 versions)
• [ ] v1.4 (TiDB Operator 1.4 versions)
• [ ] v1.3 (TiDB Operator 1.3 versions)
• [ ] v1.2 (TiDB Operator 1.2 versions)

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

• This PR is translated from:
• Other reference link(s): https://github.com/pingcap/docs-cn/pull/14569

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