add voucher info

docs/api/send-message.mdx

@@ -14,6 +14,9 @@ try {
     payload: somePayload,
     gasLimit: 10000000,
     value: 1000,
+    // prepaid: true,
+    // account: accountId,
+    // if you send message with issued voucher
   };
   // In that case payload will be encoded using meta.types.handle.input type
   let extrinsic = api.message.send(message, meta);
@@ -49,6 +52,9 @@ try {
     payload: somePayload,
     gasLimit: 10000000,
     value: 1000,
+    // prepaid: true,
+    // account: accountId,
+    // if you send message with issued voucher
   };
   // In this case payload will be encoded using `meta.types.reply.input` type.
   const extrinsic = api.message.sendReply(reply, meta);

docs/api/vouchers.md

@@ -0,0 +1,41 @@
+---
+sidebar_position: 7
+sidebar_label: Vouchers
+---
+
+# Gas vouchers
+
+Vouchers, issued by any actor empower users with gas-free interactions, enabling them to send messages to specific programs seamlessly.
+
+Use `api.voucher.issue` method to issue a new voucher for a user to be used to pay for sending messages to `program_id` program.
+
+```javascript
+  import { VoucherIssued } from '@gear-js/api';
+
+  const programId = '0x..';
+  const account = '0x...';
+  const tx = api.voucher.issue(account, programId, 10000);
+  tx.signAndSend(account, (events) => {
+    const voucherIssuedEvent = events.events.filter(({event: {method}}) => method === 'VoucherIssued') as VoucherIssued;
+    console.log(voucherIssuedEvent.toJSON());
+  })
+```
+
+Use `api.voucher.exists` method to check that the voucher exists for a particular user and program:
+
+```javascript
+const voucherExists = await api.voucher.exists(programId, accountId);
+```
+
+To send message with voucher set `prepaid` flag to `true` in the first argument of `api.message.send` and `api.message.sendReply` methods. Also it's good to specify account ID that is used to send the extrinsic to check whether the voucher exists or not.
+
+```javascript
+let massage = await api.message.send({
+  destination: destination,
+  payload: somePayload,
+  gasLimit: 10000000,
+  value: 1000,
+  prepaid: true,
+  account: accountId,
+}, meta);
+```