docs: explain Redis 8 concurrent search visibility

[mikemikimike]

Summary

Document the temporary search visibility window that Redis 8.8+ can introduce under concurrent write and query load.

Changes

• Added a Search & Indexing section describing the read-after-write behavior.
• Included a load() followed by query() example and clarified that this is indexing latency, not data loss.
• Documented the Redis server-side --search-workers 0 option for workloads that require foreground indexing, including its performance trade-off.

Validation

• uv run sphinx-build -b html docs docs/_build/html --keep-going — passed.
• uv run black --check redisvl tests — passed (249 files unchanged).
• uv run isort --check-only redisvl tests --profile black — passed.
• uv run mypy redisvl — passed.
• git diff --check — passed.

The strict documentation build with -W still reports two pre-existing cross-reference warnings in docs/user_guide/how_to_guides/mcp.md and mcp_authentication.md; the normal build completes successfully.

Compatibility

Documentation-only change. No RedisVL runtime behavior is modified.

Fixes #662.

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Note

Low Risk
Documentation-only change; no runtime or API behavior is modified.

Overview
Adds a Search Visibility Under Concurrent Writes section to the Search & Indexing concepts doc, complementing existing Redis 8.8+ notes in queries.md about background workers.

The new text explains that on Redis 8.8+, RediSearch’s default multithreaded worker pool can leave freshly written documents temporarily invisible to an immediate query() after load()—indexing latency, not data loss. It warns ingestion pipelines and read-after-write checks that query the index right after writes, and points operators to the Redis server flag --search-workers 0 when foreground indexing is required, with the performance trade-off noted.

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