Improve the Repository files documentation

What does this MR do?

I found in https://gitlab.com/gitlab-org/gitlab-ce/issues/49869 that our Repository files documentation is misleading since it advise to pass all parameters in the URL, even for POST, PUT, and DELETE requests.

This merge request solves this misleading documentation by advising to pass additional parameters in the request's body.

What are the relevant issue numbers?

Misleading documentation found thanks to https://gitlab.com/gitlab-org/gitlab-ce/issues/49869.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides