Update Mattermost slash commands docs to explain how to create a newline and...

Update Mattermost slash commands docs to explain how to create a newline and use <kbd> for user input.
See HTML5 spec: https://www.w3.org/TR/html5/text-level-semantics.html#the-kbd-element

doc/project_services/mattermost_slash_commands.md

@@ -65,7 +65,7 @@ the administrator console.
 
 ### Step 3. Create a new custom slash command in Mattermost
 
-Now that you have enabled the custom slash commands in Mattermost and opened
+Now that you have enabled custom slash commands in Mattermost and opened
 the Mattermost slash commands service in GitLab, it's time to copy these values
 in a new slash command.
 
@@ -128,20 +128,16 @@ GitLab using the Mattermost commands.
 
 ## Available slash commands
 
-The available slash commands so far are:
+The available slash commands are:
 
 | Command | Description | Example |
 | ------- | ----------- | ------- |
-| `/<trigger> issue create <title>\n<description>` | Create a new issue in the project that `<trigger>` is tied to. `<description>` is optional. | `/trigger issue create We need to change the homepage` |
-| `/<trigger> issue show <issue-number>` | Show the issue with ID `<issue-number>` from the project that `<trigger>` is tied to. | `/trigger issue show 42` |
-| `/<trigger> deploy <environment> to <environment>` | Start the CI job that deploys from one environment to another, for example `staging` to `production`. CI/CD must be [properly configured][ciyaml]. | `/trigger deploy staging to production` |
+| <kbd>/&lt;trigger&gt; issue create &lt;title&gt; **Shift** + **Enter** &lt;description&gt;</kbd> | Create a new issue in the project that `<trigger>` is tied to. `<description>` is optional. | <samp>/gitlab issue create We need to change the homepage</samp> |
+| <kbd>/&lt;trigger&gt; issue show &lt;issue-number&gt;</kbd> | Show the issue with ID `<issue-number>` from the project that `<trigger>` is tied to. | <samp>/gitlab issue show 42</samp> |
+| <kbd>/&lt;trigger&gt; deploy &lt;environment&gt; to &lt;environment&gt;</kbd> | Start the CI job that deploys from one environment to another, for example `staging` to `production`. CI/CD must be [properly configured][ciyaml]. | <samp>/gitlab deploy staging to production</samp> |
 
-To see a list of available commands that can interact with GitLab, type the
-trigger word followed by `help`:
-
-```
-/my-project help
-```
+To see a list of available commands to interact with GitLab, type the
+trigger word followed by <kbd>help</kbd>. Example: <samp>/gitlab help</samp>
 
 ![Mattermost bot available commands](img/mattermost_bot_available_commands.png)