[skip e2e]Add comments to proxy service (#13995)

Signed-off-by: Njingkl <jingjing.jia@zilliz.com>

internal/distributed/proxy/service.go

@@ -492,6 +492,7 @@ func (s *Server) GetCompactionStateWithPlans(ctx context.Context, req *milvuspb.
 	return s.proxy.GetCompactionStateWithPlans(ctx, req)
 }
 
+// GetFlushState gets the flush state of multiple segments
 func (s *Server) GetFlushState(ctx context.Context, req *milvuspb.GetFlushStateRequest) (*milvuspb.GetFlushStateResponse, error) {
 	return s.proxy.GetFlushState(ctx, req)
 }