docs(next): document workflow discovery and server actions

docs/content/docs/v5/api-reference/workflow-next/with-workflow.mdx

@@ -36,6 +36,75 @@ compiler. Remove that package from `serverExternalPackages` in your
 `next.config` to silence the warning.
 </Callout>
 
+### Workflow Discovery in Next.js
+
+`withWorkflow()` discovers workflow entrypoints from `start()` calls imported
+from `workflow/api` in your Next.js server files. In App Router projects, that
+usually means files under `app/` or `src/app/`, such as Route Handlers, pages,
+or layouts. In Pages Router projects, files under `pages/` or `src/pages/` are
+scanned.
+
+The `start()` call must be statically reachable from one of those Next.js
+entries so the compiler can find the workflow function and transform any
+`"use workflow"` and `"use step"` directives it imports.
+
+```typescript title="app/api/signup/route.ts" lineNumbers
+import { start } from "workflow/api";
+import { handleUserSignup } from "@/workflows/user-signup";
+
+export async function POST(request: Request) {
+  const { email } = await request.json();
+
+  await start(handleUserSignup, [email]);
+
+  return Response.json({ ok: true });
+}
+```
+
+<Callout type="info">
+Use `start()` in normal server-side entrypoints, including Route Handlers and
+Server Actions. Do not call workflow functions directly from those entrypoints;
+direct invocation bypasses the workflow runtime.
+</Callout>
+
+### Next.js Server Actions and `"use server"`
+
+Keep top-level `"use server"` directives on files that define Server Action
+entrypoints. Do not put a top-level `"use server"` directive in modules that are
+imported by workflow or step functions.
+
+Workflow bundles wrap imported modules during transformation. When a bundled
+module contains a top-level `"use server"` directive, Next.js can treat the
+generated wrapper as a Server Action module and fail validation with errors such
+as `Server Actions must be async functions`.
+
+Split Server Action wrappers from reusable workflow logic:
+
+```typescript title="app/actions.ts" lineNumbers
+"use server";
+
+import { start } from "workflow/api";
+import { processSignup } from "@/workflows/signup";
+
+export async function signupAction(email: string) {
+  await start(processSignup, [email]);
+}
+```
+
+```typescript title="workflows/signup.ts" lineNumbers
+export async function processSignup(email: string) {
+  "use workflow";
+
+  await createUser(email);
+}
+
+async function createUser(email: string) {
+  "use step";
+
+  return { email };
+}
+```
+
 ### Monorepos and Workspace Imports
 
 By default, Next.js detects the correct workspace root automatically. If your Next.js app lives in a subdirectory such as `apps/web` and workspace resolution is not working correctly, you can set `outputFileTracingRoot` as a workaround: