Merge branch '25144-gitlab-ce-mattermost-slash-command-for-issue-create-needs-better-documentation' into 'master'

Resolve "gitlab-ce mattermost slash command for issue create needs better documentation"

## What does this MR do?

Updates the documentation and the <kbd>help</kbd> command to be clearer, having the keys used to add a newline in chat clients (both Mattermost and Slack). 

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

* Are the available commands (via <kbd>help</kbd>) being formatted as `<code>` ?

## Why was this MR needed?

`\n` represents a new line character and doesn't communicate how the user should input the command. Also, to be correct, the documentation should use `<kbd>` instead of `<code>` for user input ([see HTML5 specification](https://www.w3.org/TR/html5/text-level-semantics.html#the-kbd-element))

## Screenshots (if relevant)

| Mattermost | Docs |
| --- | --- |
| ![image](/uploads/539526a14bfd551b7e732dd96c5b7581/image.png) | ![image](/uploads/e3eb099f86c5a32ce3b8954e72c29848/image.png) |

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [ ] All builds are passing

## What are the relevant issue numbers?

Closes #25144

See merge request !7850