[skip ci]Add comments in chap07_query_coordinator.md (#11054)

Signed-off-by: Nruiyi.jiang <ruiyi.jiang@zilliz.com>

docs/developer_guides/chap07_query_coordinator.md

@@ -17,6 +17,7 @@ type QueryCoord interface {
 	LoadCollection(ctx context.Context, req *querypb.LoadCollectionRequest) (*commonpb.Status, error)
   // ReleaseCollection notifies Proxy to release a collection's data
 	ReleaseCollection(ctx context.Context, req *querypb.ReleaseCollectionRequest) (*commonpb.Status, error)
+  // ShowPartitions notifies RootCoord to list all partition names and other info in the collection
 	ShowPartitions(ctx context.Context, req *querypb.ShowPartitionsRequest) (*querypb.ShowPartitionsResponse, error)
 	LoadPartitions(ctx context.Context, req *querypb.LoadPartitionsRequest) (*commonpb.Status, error)
 	ReleasePartitions(ctx context.Context, req *querypb.ReleasePartitionsRequest) (*commonpb.Status, error)