Summary

Document that FTS indexes accept dotted paths to text leaves inside struct columns, on both sync and async APIs.

Changes

• Add a "Nested field paths" section to the FTS index page covering supported paths, async equivalence, and the error cases for struct containers, non-text leaves, and missing paths.
• Add a testable fts_index_nested snippet that creates an index on payload.text and runs MatchQuery / PhraseQuery against it.
• Export the generated snippet from the indexing snippets module so the docs page can import it.

Context

The upstream PR adds regression tests covering nested FTS field paths (sync and async match, phrase, and hybrid queries) and locks in the error boundary introduced in lancedb/lancedb#3408. The behavior was previously undocumented.