docs(retrievers): document BM25Plus variant for BM25Retriever

[samanta-sc]

Overview

Documents the optional BM25Plus variant supported by BM25Retriever.

Adds a short explanation of when BM25Plus may be useful and a minimal example showing how to enable it. The default behavior of BM25Retriever remains unchanged.

Type of change

Type: Update existing documentation

Related issues/PRs

• GitHub issue: N/A
• Feature PR: feat(retrievers): Add BM25Plus variant support to BM25Retriever langchain-community#488

Checklist

• ✅ I have read the contributing guidelines
• ✅ I have tested my changes locally using docs dev
• ✅ All code examples have been tested and work correctly
• ✅ I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed
  (Not needed — existing page updated only)

Additional notes

This documentation corresponds to an opt-in feature added in the linked code PR. No existing behavior is changed.