ai fts: improve full-text index docs and scoring details

[zhaoshangzi]

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What is changed, added or deleted? (Required)

Expand the full-text search guide with practical details: clarify that the STANDARD parser lowercases text for case-insensitive matching; add a "Managing full-text indexes" section explaining index naming, viewing (SHOW INDEX / INFORMATION_SCHEMA), dropping, and examples for CREATE/ALTER/CREATE FULLTEXT INDEX; add sections on tokenization and multi-word search semantics (fts_match_word uses tokenization and OR semantics, no phrase or prefix matching), explain effect of repeated terms on BM25 scoring, and document the BM25Tanvity relevance algorithm with the standard k1=1.2 and b=0.75 defaults.

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