Document callback-triggered cancellation

include/git2/indexer.h

@@ -25,7 +25,11 @@ typedef struct git_transfer_progress {
 
 
 /**
- * Type for progress callbacks during indexing
+ * Type for progress callbacks during indexing.  Return a value less than zero
+ * to cancel the transfer.
+ *
+ * @param stats Structure containing information about the state of the transfer
+ * @param payload Payload provided by caller
  */
 typedef int (*git_transfer_progress_callback)(const git_transfer_progress *stats, void *payload);