i18n(ja): fix mistranslation of the database term sargable

[yahonda]

What is changed, added or deleted? (Required)

"Sargable" (Search ARGument-able) is a real database term describing whether a condition can use an index. It appears twice in the corpus, and the second occurrence links to https://en.wikipedia.org/wiki/Sargable, confirming it's intentional jargon, not a typo. The Japanese Wikipedia has a matching article titled 検索引数可能, which is the correct established Japanese term.

Both existing ja occurrences mistranslated the term differently:

• explain-overview.md: italic emphasis wrapped only part of the term (*検索引数*可能 instead of *検索引数可能*), and a particle was dropped in the same sentence.
• explain-partitions.md: "non-sargable" was rendered as 非検索可能 ("non-searchable"), dropping the "argument" component and changing the meaning. Fixed to 検索引数可能でない (negated with でない). Also restored a dropped に particle and と particle in the same sentence.

The Wikipedia link target is left unchanged (en.wikipedia.org) per this corpus's existing convention — no ja translation anywhere localizes a Wikipedia link (0 ja.wikipedia.org links vs 56 en.wikipedia.org links corpus-wide).

Two more dropped-particle defects found while reviewing the same two Note/list blocks:

• explain-overview.md: missing を before "比較することをお勧めします" ("it is recommended to compare data of the same type and [character set and collation]").
• explain-partitions.md: missing は after the operator name in "The operator PartitionUnion_21 unions the results from accessing each partition."

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

• 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)?

N/A

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• Delete files
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• Might cause conflicts after applied to another branch

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