Preserve and document legacy Bitmap distributed merge

[Xuanwo]

Summary

Keep the existing Bitmap distributed merge workflow compatible while documenting the new canonical segment path.

Part of #6862.

Requirements

• Keep merge_index_metadata(index_uuid, "BITMAP") functional.
• Keep BitmapParameters.shard_id functional for the legacy path.
• Document that legacy index_uuid is a shared merge directory / job id, not a physical segment id.
• Document the canonical vector-style Bitmap segment workflow as the preferred new path.

Acceptance criteria

• Legacy Python distributed Bitmap tests continue to pass.
• Documentation distinguishes legacy shard-file merge from canonical segment lifecycle.
• New examples do not require shard_id.
• No default behavior changes for ordinary Bitmap index creation.