Update search docs with gaps identified

[prrao87]

Summary

Close five gaps surfaced by the docs-audit run on the search area, all grounded in the lancedb OSS source and Enterprise serving layer:

• Full-text search: new "Keeping the index up to date" section explaining post-ingest optimize(), the unindexed-tail flat-scan fallback, and fast_search(). Reuses the existing PyFtsIncrementalIndex snippet.
• Hybrid search: new "Query controls" section covering with_row_id / withRowId, distance_range / distanceRange, and prefilter vs. postfilter. Verified against LanceHybridQueryBuilder._create_query_builders and TS QueryBase.
• Multivector search: callout that brute-force scales with rows × vectors-per-row, that LanceDB Enterprise applies a ~10× stricter brute-force KNN threshold for multivector columns (per sophon's MULTI_VECTOR_ROW_MULTIPLIER), and that OSS has no equivalent guard. The ColBERT example now builds the index instead of skipping it.
• Optimize Query Performance: new "Analyzing non-vector queries" section showing explain_plan / analyze_plan on FTS and hybrid queries, including the reranker-label + sub-plan format hybrid emits.
• SQL overview: new "Inspecting query plans" section for EXPLAIN / EXPLAIN ANALYZE / EXPLAIN VERBOSE over the FlightSQL endpoint, with a pointer back to the optimize page.

No new tests or snippet regeneration needed — the only snippet import added (PyFtsIncrementalIndex) was already generated.

Test plan

• Mintlify preview renders the new sections without import errors
• Anchor link /search/optimize-queries#analyzing-non-vector-queries resolves from hybrid-search.mdx
• CodeGroup blocks render Python / TypeScript / Rust tabs as expected

[mintlify]

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Project	Status	Preview	Updated (UTC)
lancedb-bcbb4faf	🟢 Ready	View Preview	May 4, 2026, 3:06 PM

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