restructure migration guide

[patrickthornton]

Unifies around

let page = await client.actions.list();
while (page.hasNextPage()) {
    console.log(page);
    page = await page.getNextPage();
}

in documentation, with

const response = await client.actions.list();
for await (const item of response) {
    console.log(item);
}

as a listed alternative.