gRPC Stream class docs

[olegbespalov]

What?

This change fixes the missed gRPC Stream class signature and improves its documentation.

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.

• I have made my changes in the docs/sources/next folder of the documentation.
• I have reflected my changes in the docs/sources/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant folders of the two previous k6 versions of the documentation (if still applicable to previous versions).

Related PR(s)/Issue(s)

Spotted during https://community.grafana.com/t/k6-server-grpc-streaming-with-accesstoken-metadata/117879

[heitortsergent]

Left some small edits, other than that everything looks good! Might need to run prettier again on the files to fix the tables if you accept them. 🙇

[olegbespalov]

@heitortsergent thanks! 👍 I hope I've applied all the suggestions in a separate commit. Could you please have a final look and re-approve it? ☺️

[heitortsergent]

LGTM! 🚀