[#105] balance: Add docs and fix naming

`CreateToken` method should not be public.
Fixed notification field names in `Mint` and `Burn`.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

balance/doc.go

@@ -1,2 +1,79 @@
-// Balance contract description.
+/*
+Balance contract is a contract deployed in NeoFS side chain.
+
+Balance contract stores all NeoFS account balances. It is NEP-17 compatible
+contract so in can be tracked and controlled by N3 compatible network
+monitors and wallet software.
+
+This contract is used to store all micro transactions in the sidechain, such as
+data audit settlements or container fee payments. It is inefficient to make such
+small payment transactions in main chain. To process small transfers, balance
+contract has higher (12) decimal precision than native GAS contract.
+
+NeoFS balances are synchronized with main chain operations. Deposit produce
+minting of NEOFS tokens in Balance contract. Withdraw locks some NEOFS tokens
+in special lock account. When NeoFS contract transfers GAS assets back to the
+user, lock account is destroyed with burn operation.
+
+Contract notifications
+
+Transfer notification. This is NEP-17 standard notification.
+
+  Transfer:
+    - name: from
+      type: Hash160
+    - name: to
+      type: Hash160
+    - name: amount
+      type: Integer
+
+TransferX notification. This is enhanced transfer notification with details.
+
+  TransferX:
+    - name: from
+      type: Hash160
+    - name: to
+      type: Hash160
+    - name: amount
+      type: Integer
+    - name: details
+      type: ByteArray
+
+Lock notification. This notification is produced when Lock account has been
+created. It contains information about main chain transaction that produced
+asset lock, address of lock account and NeoFS epoch number until lock account
+is valid. Alphabet nodes of the Inner Ring catch notification and initialize
+Cheque method invocation of the NeoFS contract.
+
+  Lock:
+    - name: txID
+      type: ByteArray
+    - name: from
+      type: Hash160
+    - name: to
+      type: Hash160
+    - name: amount
+      type: Integer
+    - name: until
+      type: Integer
+
+Mint notification. This notification is produced when user balance is
+replenished from deposit in the main chain.
+
+  Mint:
+   - name: to
+     type: Hash160
+   - name: amount
+     type: Integer
+
+
+Burn notification. This notification is produced after user balance is reduced
+when NeoFS contract transferred GAS assets back to the user.
+
+  Burn:
+    - name: from
+      type: Hash160
+    - name: amount
+      type: Integer
+*/
 package balance