Docs updates to reranker section from audit workflow

[prrao87]

Reranking docs gaps — fixes from audit run 20260504T152409Z

Addresses the six gaps flagged in the docs-audit report for the reranking area, cross-checked against the lancedb source repo.

Overview page

• Multi-vector example: added .with_row_id(True) on each search call and a warning explaining that deduplicate=True raises ValueError without _rowid (which is opt-in, so this is the default failure mode). Also calls out that RRFReranker.rerank_multivector(...) requires _rowid regardless of the flag.
• SDK coverage: added a note after the Supported Rerankers table clarifying that CohereReranker / CrossEncoderReranker / ColbertReranker are Python-only — TypeScript and Rust currently expose only the generic Reranker interface and RRFReranker.

Custom rerankers page

• Intro: rewritten to explain which methods to override (rerank_hybrid() required; rerank_vector() / rerank_fts() optional) and to flag that the base class raises NotImplementedError if you call rerank on a single-modality path you didn't override.
• Return-shape contract: added a short paragraph to the Interface section stating the actual rule — your reranker must attach a _relevance_score column sorted descending, otherwise LanceDB rejects the table or .limit(...) returns rows in the wrong order.

Evaluation page

• Score-based bullet: reframed so RRF is mentioned as the default and linear combination is one option, rather than implying linear combination is the only score-based path.
• Default reranker: added an info callout noting that hybrid .rerank() defaults to RRFReranker().
• normalize parameter: added a tip documenting normalize="score" (default) vs normalize="rank" and why it matters when evaluating score-based rerankers.

No test files changed, so no snippet regeneration was done as a result.

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