Documentation: Describe the new useFetch() behavior

[leoboyerbx]

Describe the feature

Since Nuxt 3.2, useFetch automatically passes request data to api call in an SSR context. The steps described in the docs aren't needed anymore: https://nuxt.com/docs/getting-started/data-fetching#example-pass-client-headers-to-the-api

I think we shouldn't remove this section right away, but I propose to add a small alert in the docs saying that this change is not necessary anymore.

Additional information

• [X] Would you be willing to help implement this feature?
• [ ] Could this feature be implemented as a module?

Final checks

• [X] Read the contribution guide.
• [X] Check existing discussions and issues.

[leoboyerbx]

AFAIK, this should still be needed if requesting an external API. This should be clear in the docs whenever using useRequestHeaders() is needed or not