document expand button feature

Author: devalog

fern/products/docs/pages/component-library/default-components/code-blocks.mdx

@@ -121,8 +121,10 @@ a `maxLines` attribute after the language identifier. The
 `maxLines` attribute should be a number representing the maximum 
 number of lines to display. By default, the code block will display up to 20 lines.
 
+When you use `maxLines`, an expand button automatically appears on hover in the top-right corner, allowing users to view the full code content in an expanded overlay that displays over the page.
+
 <Tabs>
-  <Tab title="Example">
+  <Tab title="Example: maxLines=10">
     ```python maxLines=10
     def is_prime(num):
         """Check if a number is prime."""
@@ -182,6 +184,15 @@ number of lines to display. By default, the code block will display up to 20 lin
   </Tab>
 </Tabs>
 
+To hide the expand button or add custom styling, target the `.fern-expand-button` selector:
+
+```css
+/* Hide the expand button */
+.fern-expand-button {
+  display: none;
+}
+```
+
 ## Wrap overflow
 
 By default, long lines that exceed the width of the code block become scrollable: