[#29801] Add "Slack Slash Commands" to API docs

- Reorder slash command api requests, include slack response example
- Case fix and added changelog

changelogs/unreleased/29801-add-slash-slack-commands-to-api-doc.yml

+---
+title: Add Slack slash command api to services documentation and rearrange order and
+  cases
+merge_request: 10757
+author: TM Lee

doc/api/services.md

@@ -490,41 +490,98 @@ Remove all previously JIRA settings from a project.
 DELETE /projects/:id/services/jira
 ```
-## Mattermost Slash Commands
-Ability to receive slash commands from a Mattermost chat instance.
-### Create/Edit Mattermost Slash Command service
-Set Mattermost Slash Command for a project.
+## Slack slash commands
+Ability to receive slash commands from a Slack chat instance.
+### Get Slack slash command service settings
+Get Slack slash command service settings for a project.
 ```
-PUT /projects/:id/services/mattermost-slash-commands
+GET /projects/:id/services/slack-slash-commands
+```
+
+Example response:
+
+```json
+{
+  "id": 4,
+  "title": "Slack slash commands",
+  "created_at": "2017-06-27T05:51:39-07:00",
+  "updated_at": "2017-06-27T05:51:39-07:00",
+  "active": true,
+  "push_events": true,
+  "issues_events": true,
+  "merge_requests_events": true,
+  "tag_push_events": true,
+  "note_events": true,
+  "build_events": true,
+  "pipeline_events": true,
+  "properties": {
+    "token": "9koXpg98eAheJpvBs5tK"
+  }
+}
+```
+
+### Create/Edit Slack slash command service
+
+Set Slack slash command for a project.
+
+```
+PUT /projects/:id/services/slack-slash-commands
 ```
 Parameters:
 | Attribute | Type | Required | Description |
 | --------- | ---- | -------- | ----------- |
-| `token` | string | yes | The Mattermost token |
-### Delete Mattermost Slash Command service
-Delete Mattermost Slash Command service for a project.
+| `token` | string | yes | The Slack token |
+### Delete Slack slash command service
+Delete Slack slash command service for a project.
 ```
-DELETE /projects/:id/services/mattermost-slash-commands
+DELETE /projects/:id/services/slack-slash-commands
 ```
-### Get Mattermost Slash Command service settings
-Get Mattermost Slash Command service settings for a project.
+## Mattermost slash commands
+
+Ability to receive slash commands from a Mattermost chat instance.
+
+### Get Mattermost slash command service settings
+Get Mattermost slash command service settings for a project.
 ```
 GET /projects/:id/services/mattermost-slash-commands
 ```
+### Create/Edit Mattermost slash command service
+
+Set Mattermost slash command for a project.
+
+```
+PUT /projects/:id/services/mattermost-slash-commands
+```
+
+Parameters:
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `token` | string | yes | The Mattermost token |
+
+
+### Delete Mattermost slash command service
+
+Delete Mattermost slash command service for a project.
+
+```
+DELETE /projects/:id/services/mattermost-slash-commands
+```
+
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