Document multiline help text for app settings (#228)

## 💸 TL;DR
<!-- What's the three sentence summary of purpose of the PR -->
I ran an audit of what was missing from our changelog in our docs and
had codex run a first pass.

Shows how to add multiline instructions beneath app settings.

Author: vdikransamarjian

docs/capabilities/server/settings-and-secrets.mdx

@@ -79,6 +79,29 @@ Define settings in your `devvit.json` file under the `settings` object. Settings
 After defining settings in `devvit.json`, you must build your app (`npm run dev`) before you can set secrets via the CLI.
 :::
 
+### Help text
+
+Add optional `helpText` to explain a setting. In the installation settings UI, line breaks are preserved. Use `\n` for a new line and `\n\n` for a blank line in JSON:
+
+```json title="devvit.json"
+{
+  "settings": {
+    "subreddit": {
+      "welcomeMessage": {
+        "type": "string",
+        "label": "Welcome Message",
+        "defaultValue": "Welcome to the community!",
+        "helpText": "Shown to new members.\nKeep the message short.\n\nDo not include private information."
+      }
+    }
+  }
+}
+```
+
+Help text is plain text. Use line breaks to separate instructions, and avoid relying on HTML, Markdown, or repeated spaces for formatting. Omit `helpText` or use an empty string to display no help text.
+
+Individual global and subreddit settings support `helpText`. The example above shows its presentation in installation settings. Help text provides guidance; use `defaultValue` to set an initial value and `validationEndpoint` to validate input.
+
 ## Setting types
 
 The following setting types are supported:

versioned_docs/version-0.14/capabilities/server/settings-and-secrets.mdx

@@ -79,6 +79,29 @@ Define settings in your `devvit.json` file under the `settings` object. Settings
 After defining settings in `devvit.json`, you must build your app (`npm run dev`) before you can set secrets via the CLI.
 :::
 
+### Help text
+
+Add optional `helpText` to explain a setting. In the installation settings UI, line breaks are preserved. Use `\n` for a new line and `\n\n` for a blank line in JSON:
+
+```json title="devvit.json"
+{
+  "settings": {
+    "subreddit": {
+      "welcomeMessage": {
+        "type": "string",
+        "label": "Welcome Message",
+        "defaultValue": "Welcome to the community!",
+        "helpText": "Shown to new members.\nKeep the message short.\n\nDo not include private information."
+      }
+    }
+  }
+}
+```
+
+Help text is plain text. Use line breaks to separate instructions, and avoid relying on HTML, Markdown, or repeated spaces for formatting. Omit `helpText` or use an empty string to display no help text.
+
+Individual global and subreddit settings support `helpText`. The example above shows its presentation in installation settings. Help text provides guidance; use `defaultValue` to set an initial value and `validationEndpoint` to validate input.
+
 ## Setting types
 
 The following setting types are supported: