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

doc/api/issues.md

@@ -323,7 +323,7 @@ POST /projects/:id/issues
 | `assignee_id`   | integer | no  | The ID of a user to assign issue |
 | `milestone_id`  | integer | no  | The ID of a milestone to assign issue |
 | `labels`        | string  | no  | Comma-separated label names for an issue  |
-| `created_at`    | string  | no  | Date time string, ISO 8601 formatted, e.g. `2016-03-11T03:45:40Z` |
+| `created_at`    | string  | no  | Date time string, ISO 8601 formatted, e.g. `2016-03-11T03:45:40Z` (requires admin or project owner rights) |
 | `due_date`      | string  | no  | Date time string in the format YEAR-MONTH-DAY, e.g. `2016-03-11` |
 
 ```bash
@@ -384,7 +384,7 @@ PUT /projects/:id/issues/:issue_id
 | `milestone_id`  | integer | no  | The ID of a milestone to assign the issue to |
 | `labels`        | string  | no  | Comma-separated label names for an issue  |
 | `state_event`   | string  | no  | The state event of an issue. Set `close` to close the issue and `reopen` to reopen it |
-| `updated_at`    | string  | no  | Date time string, ISO 8601 formatted, e.g. `2016-03-11T03:45:40Z` |
+| `updated_at`    | string  | no  | Date time string, ISO 8601 formatted, e.g. `2016-03-11T03:45:40Z` (requires admin or project owner rights) |
 | `due_date`      | string  | no  | Date time string in the format YEAR-MONTH-DAY, e.g. `2016-03-11` |
 
 ```bash