[#275] audit: Document contract storage model

Add comments (outside the docs) with key-value storage structure and
some explanations. The memory mode will allow to more precisely
understand and migrate the contract storage.

Signed-off-by: Leonard Lyubich <[email protected]>

audit/doc.go

@@ -21,3 +21,28 @@ they make a list and get these AuditResultStructures from the audit contract.
 Audit contract does not produce notifications to process.
 */
 package audit
+
+/*
+Contract storage model.
+
+# Summary
+Key-value storage format:
+ - 'notary' -> bool
+   is notary mode disabled
+- 'netmapScriptHash' -> interop.Hash160
+   Netmap contract reference
+ - [64]byte -> []byte
+   Data audit results encoded into NeoFS API binary protocol format. Results are
+   identified by triplet concatenation:
+    1. 8-byte little-endian unsigned integer NeoFS epoch when audit was performed
+    2. 32-byte identifier of the NeoFS container under audit
+    3. 24-byte prefix of SHA-256 hash of the auditor's (Inner Ring) public key
+
+# Setting
+Contract can be deployed in notary and notary-disabled mode.
+
+To handle some events, the contract refers to other contracts.
+
+# Audit history
+Contracts stores results of the NeoFS data audits performed by the Inner Ring.
+*/