docs: Explain the purpose of updateId (#1318)

Author: lorensr

packages/client/src/workflow-options.ts

@@ -66,6 +66,14 @@ export function compileWorkflowOptions<T extends WorkflowOptions>(options: T): W
 }
 
 export interface WorkflowUpdateOptions {
+  /**
+   * The Update Id, which is a unique-per-Workflow-Execution identifier for this Update.
+   *
+   * We recommend setting it to a meaningful business ID or idempotency key (like a request ID) passed from upstream. If
+   * it is not provided, it will be set to a random string by the Client. If the Server receives two Updates with the
+   * same Update Id to a Workflow Execution with the same Run Id, the second Update will return a handle to the first
+   * Update.
+   */
   readonly updateId?: string;
 }