Meilisearch Cloud: Include documentation for custom user bindings

[fabio-nettis]

Pull Request

Related issue

Fixes meilisearch/meilisearch#4309

What does this PR do?

This PR includes documentation for the X-MS-USER-ID header. This header allows to manually set the user id associated with a search and subsequent requests to the /event endpoint. Based on the feedback of this issue.

PR checklist

Please check if your PR fulfills the following requirements:

• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• Have you read the contributing guidelines?
• Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

[guimachiavelli]

@fabio-nettis Thanks for the PR, and great job in creating a new <Capsule>.

A couple of comments on my side:

• hard line breaks often create issues when rendering markdown into HTML, so we prefer to avoid them in regular text
• I suggested a slightly more informative title for the capsule
• I also made a few suggestions to make the new capsule text a bit more idiomatic

[guimachiavelli]

Thanks, LGTM!