docs: update documentation for stats

fs/accounting/stats_groups.go

@@ -63,10 +63,16 @@ func init() {
 		Fn:    remoteStats,
 		Title: "Returns stats about current transfers.",
 		Help: `
-This returns all available stats
+This returns all available stats:
 
 	rclone rc core/stats
 
+If group is not provided then summed up stats for all groups will be
+returned.
+
+Parameters
+- group - name of the stats group (string)
+
 Returns the following values:
 
 ` + "```" + `