[Serve] [Docs] Add note about DAGDriver redirect

[shrekris-anyscale]

Why are these changes needed?

The DAGDrvier does an HTTP redirect when processing requests. If users query a DAGDriver endpoint with curl, the request won't return anything unless they include the -L flag, which makes curl follow redirects.

This change adds a note about this.

Related issue number

Closes #36773.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • This change relies on existing tests.