[Durable Objects] Add example to transactionSync().

Author: kentonv

src/content/docs/durable-objects/api/storage-api.mdx

@@ -207,6 +207,41 @@ The `put()` method returns a `Promise`, but most applications can discard this p
 
   * The callback must complete synchronously, that is, it should not be declared `async` nor otherwise return a Promise. Only synchronous storage operations can be part of the transaction. This is intended for use with SQL queries using [`ctx.storage.sql.exec()`](#sqlexec), which complete sychronously.
 
+```ts
+  // Transfer money from one account to another. It's
+  // important that either both accounts are updated
+  // or neither is!
+  async transfer(amount, fromAcct, toAcct) {
+    // Open a transaction, so either all changes apply or
+    // none do.
+    ctx.storage.transactionSync(() => {
+      // Fetch both account balances.
+      let account1 = ctx.storage.sql.exec(
+          "SELECT balance FROM accounts WHERE id = ?", fromAcct)
+          .one();
+      let account2 = ctx.storage.sql.exec(
+          "SELECT balance FROM accounts WHERE id = ?", toAcct)
+          .one();
+
+      // Verify there's enough money in the account.
+      if (account1.balance < amount) {
+        // Throwing an error rolls back the transaction.
+        // (Though in this example there's no changes to roll
+        // back yet.)
+        throw new Error("Not enough money in account.");
+      }
+
+      // Update both account balances.
+      ctx.storage.sql.exec(
+          "UPDATE accounts SET balance = ? WHERE id = ?",
+          account1.balance - amount, fromAcct);
+      ctx.storage.sql.exec(
+          "UPDATE accounts SET balance = ? WHERE id = ?",
+          account2.balance + amount, toAcct);
+    });
+  }
+```
+
 ### sync
 
 * `sync()` : <Type text='Promise' />