[DOC] gRPC Search: FunctionScoreQuery fields + knn cross-link

[Nishieee]

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Update _api-reference/grpc-apis/search.md:

1. FunctionScoreQuery - function_score is listed in Supported queries (compound) and in the QueryContainer table, but there is no field subsection (unlike BoolQuery / ConstantScoreQuery). Add #### FunctionScoreQuery fields under compound queries, matching page style and the FunctionScoreQuery message @ 1.2.0.

2. knn cross-link - Supported queries and the table include knn, but Specialized query fields only documents ScriptQuery and HybridQuery. Add a short note linking to k-NN (gRPC) so readers match the reciprocal link from knn.md back to this page.

Version: OpenSearch 3.x (gRPC Search; protobufs 1.2.0 as referenced on the page).

What other resources are available?

• Protobuf: FunctionScoreQuery
• Related: k-NN (gRPC)