Extend JSON function docs

[dveeden]

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For v8.1 and earlier: The whole validation document with JSON_SCHEMA_VALID() can be removed as this was introduced in v8.2 (see https://github.com/pingcap/docs/pull/17684 )

[ti-chi-bot[bot]]

Skipping CI for Draft Pull Request. If you want CI signal for your change, please convert it to an actual PR. You can still manually trigger a test run with /test all

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@mjonss: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[LGTM Timeline notifier]

Timeline:

• 2024-06-24 11:23:51.040632121 +0000 UTC m=+632357.526120946: :ballot_box_with_check: agreed by hfxsd.
• 2024-07-10 06:23:28.286270008 +0000 UTC m=+425105.521504122: :ballot_box_with_check: agreed by qiancai.

[hfxsd]

/approve

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18164.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18165.