release-8.5: clarify IMPORT INTO format detection by platform and version

[alastori]

What is changed, added or deleted? (Required)

This PR aligns the IMPORT INTO format documentation on release-8.5 with the behavior released in TiDB v8.5.7 and the current TiDB Cloud behavior:

• Explains that an explicit FORMAT clause overrides file-extension detection.
• Qualifies automatic format detection as available in TiDB Self-Managed starting from v8.5.7.
• Preserves the v8.5.6-and-earlier behavior in which omitting FORMAT uses CSV.
• Uses platform-specific content so the TiDB Cloud documentation does not expose a Self-Managed patch-version qualifier.
• Documents case-insensitive detection for .csv, .sql, and .parquet files in the applicable versions and platform.
• Lists the supported compression suffixes ignored during detection.
• Clarifies that a missing or unrecognized extension falls back to CSV.
• Explains that wildcard imports use one format for all matched files and avoids claiming that mixed formats fail during precheck.

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

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

• This PR is translated from:
• Other reference link(s):
  • import into: automatic file type recognition based on file name characteristics tidb#59543
  • importinto: add test for auto detect source file format tidb#63884
  • import into: automatic file type recognition based on file name characteristics (#59543) tidb#66058
  • IMPORT INTO autodetect file format and wildcards for file path #21197
  • cloud: clarify IMPORT INTO wildcard path matching #23488

Validation

• ./scripts/markdownlint sql-statements/sql-statement-import-into.md
• python3 scripts/file-format-lint.py sql-statements/sql-statement-import-into.md
• python3 scripts/check-manual-line-breaks.py sql-statements/sql-statement-import-into.md
• git diff --check
• Verified that the platform filters retain the v8.5.7 qualifier and earlier-version fallback for TiDB Self-Managed, while TiDB Cloud retains the current automatic-detection wording.
• Local documentation review gate: APPROVED, with no findings.
• Direct smoke test against TiDB v8.5.7 and an S3-compatible MinIO endpoint. This validates the TiDB Self-Managed engine behavior for case-insensitive CSV, SQL, and Parquet detection; .gz, .gzip, .zstd, .zst, and .snappy suffixes; missing and unrecognized extension fallback; explicit FORMAT override; and homogeneous wildcard imports.
• A mixed .csv and .sql wildcard created an import job and failed while parsing the SQL file as CSV. It did not fail during precheck, and the target table remained empty.
• A live TiDB Cloud cluster was not exercised in this smoke test.

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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

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[alastori]

@JQWong7, could you review this documentation update for the IMPORT INTO automatic format-detection behavior implemented in pingcap/tidb#59543?

I smoke-tested the release-8.5 backport on TiDB v8.5.7, including explicit FORMAT, case-insensitive file and compression extensions, CSV fallback, and wildcard imports. The mixed-format case failed during the import rather than during precheck, so this draft avoids that earlier documentation claim.