What does this MR do and why?

Currently, the documentation for the API request to create a new issue (POST /projects/:id/issues) says:

labels — Comma-separated label names for an issue.

It doesn't explain what happens if the label doesn't already exist. This MR adds documentation describing the current behavior.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist.

Screenshots or screen recordings

n/a

How to set up and validate locally

n/a