[fix] correct typos / wrong descriptions / stray markdown in SQL data-type and aggregate-function docs

[boluor]

Summary

Fixes ten small documentation issues across sql-manual/basic-element/ and sql-manual/sql-functions/aggregate-functions/. Each item below is independent.

• STRING.md — drop the spurious STRING (M) from the type description (both the frontmatter description and the body). STRING does not take a length parameter.
• variables.md — the version_comment row showed different values in the Value vs Default_Value columns (doris0.0.0--de61c5823 vs doris-0.0--de61c5823). Both columns now use the canonical doris-0.0.0--de61c58223 form already used in the zh-CN translation.
• ai-agg.md — typo default_ai_resoure → default_ai_resource.
• array-agg.md — remove a stray | 1 | ["a","b"] | table row that sat outside the closing code fence and rendered as literal text.
• bitmap-intersect.md — collapse two consecutive ## Example headings into one.
• bool-or.md — the Return Value paragraph said "TRUE when all non-NULL values exist", which is the logical AND semantics. Corrected to "TRUE when at least one non-NULL value is TRUE".
• covar-samp.md — remove an orphan duplicated result-table block plus an extra closing code fence that appeared after the legitimate one.
• histogram.md — the lower and upper bucket field descriptions were swapped (lower: Upper bound, upper: Lower bound). Corrected.
• JSON.md — two consecutive bullets both began with CAST(string AS JSON):. Merged into one bullet that keeps the content of both.
• percentile_reservoir.md — the third example's query and result header used percentile(...) instead of percentile_reservoir(...) (looked like copy-paste from percentile.md).

Test plan

• CI doc build passes
• Spot-check the affected pages render as expected (STRING.md description, variables.md version_comment row, single ## Example in bitmap-intersect, clean result tables in array-agg / covar-samp, corrected bucket field descriptions in histogram, merged bullet in JSON.md, percentile_reservoir in the third example)