docs: explain post-write search visibility

[pttydou]

Summary

• Explain the Redis 8.8+ search visibility window under concurrent writes and queries when background search workers are enabled.
• Clarify that the data remains in the keyspace and that the reported 1–3 ms convergence is an observation, not a latency guarantee.
• Document the Redis server-side --search-workers 0 option and its indexing-throughput trade-off.

Validation

• uv run --frozen sphinx-build -b html docs docs/_build/html --keep-going (passed; 12 pre-existing warnings in untouched files)
• uv run --frozen codespell docs/concepts/search-and-indexing.md
• git diff --check

Documentation-only (auto:documentation).

Fixes #662.

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Note

Low Risk
Documentation-only change with no runtime or API impact.

Overview
Adds a Search Visibility After Writes section to the search-and-indexing concept doc, describing how Redis 8.8+ with background search-workers can briefly hide newly written documents from immediate queries (e.g. right after load()).

The new text frames this as indexing lag rather than data loss, cites issue #662’s ~1–3 ms observations without treating them as an SLA, and documents the Redis server flag --search-workers 0 for strict read-after-write at the cost of background indexing throughput.

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