add screenshot, more copy

Author: alicenstar

docs/vendor/enterprise-portal-configure.mdx

@@ -148,7 +148,13 @@ To create channel-specific installation instructions:
 
 1. In the MDX template editor, write your custom installation instructions using Markdown, template variables, and MDX components.
 
-   The editor provides autocomplete for available template variables and components. Press `Ctrl+Space` to see suggestions.
+   The editor provides autocomplete for available template variables and components. Start typing `{` for template variables or `<` for MDX components to see suggestions.
+
+   ![MDX template editor](/images/vendor-portal-custom-instructions-editing.png)
+
+   [View a larger version of this image](/images/vendor-portal-custom-instructions-editing.png)
+
+1. Click **Save changes** to save your custom installation instructions.
 
 #### Template Variables