docs(examples): with-iron-session logout must be called via POST (#22240)

1. Logout is better called in a POST request (avoids caching in most situations + avoids weird CSRF "logout attacks", i.e. can't be logged out by clicking on an email link)
2. We should wait for logout to complete before mutating the user, otherwise some weird race conditions might prevent swr to catch the right logged out state

examples/with-iron-session/components/Header.js

@@ -43,7 +43,10 @@ const Header = () => {
                   href="/api/logout"
                   onClick={async (e) => {
                     e.preventDefault()
-                    await mutateUser(fetchJson('/api/logout'))
+                    mutateUser(
+                      await fetchJson('/api/logout', { method: 'POST' }),
+                      false
+                    )
                     router.push('/login')
                   }}
                 >