[#170] audit: Refactor and document package functionality

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

audit/doc.go

@@ -0,0 +1,24 @@
+/*
+Package audit provides features to process data audit in NeoFS system.
+
+Result type groups values which can be gathered during data audit process:
+	var res audit.Result
+	res.ForEpoch(32)
+	res.ForContainer(cnr)
+	// ...
+	res.Complete()
+
+Result instances can be stored in a binary format. On reporter side:
+	data := res.Marshal()
+	// send data
+
+On receiver side:
+	var res audit.Result
+	err := res.Unmarshal(data)
+	// ...
+
+Using package types in an application is recommended to potentially work with
+different protocol versions with which these types are compatible.
+
+*/
+package audit