Merge branch 'issue-created_at-docs' into 'master'

Issue created at docs

## What does this MR do?

Clarifies `created_at` for creating issues via the API.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

Docs were unclear.

## What are the relevant issue numbers?

Closes #18458 and closes #19863.

## Screenshots (if relevant)

N/A

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5967