[skip ci]Add some comments in chap07_query_coordinator.md (#11386)

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

docs/developer_guides/chap07_query_coordinator.md

@@ -249,6 +249,7 @@ type QueryNode interface {
 	LoadSegments(ctx context.Context, req *querypb.LoadSegmentsRequest) (*commonpb.Status, error)
 	// ReleaseCollection notifies Proxy to release a collection's data
 	ReleaseCollection(ctx context.Context, req *querypb.ReleaseCollectionRequest) (*commonpb.Status, error)
+	// ReleasePartitions notifies Proxy to release partitions' data
 	ReleasePartitions(ctx context.Context, req *querypb.ReleasePartitionsRequest) (*commonpb.Status, error)
 	ReleaseSegments(ctx context.Context, req *querypb.ReleaseSegmentsRequest) (*commonpb.Status, error)
 	GetSegmentInfo(ctx context.Context, req *querypb.GetSegmentInfoRequest) (*querypb.GetSegmentInfoResponse, error)