docs: clarify multi-query wording

[egolearner]

Summary

• clarify the hybrid retrieval release highlights in the English and Chinese READMEs
• describe the public behavior as a single query call instead of presenting the internal MultiQuery name as a public API
• mention scalar filtering and reranking explicitly

Validation

• git diff --check
• commit hooks passed; code format hooks skipped because this is a documentation-only change
• CI skipped via the [skip ci] commit marker

Fix #562