docs(BREAKING): include breaking changes to button border radius

BREAKING.md

@@ -15,11 +15,16 @@ This is a comprehensive list of the breaking changes introduced in the major ver
 ## Version 9.x
 
 - [Components](#version-9x-components)
+  - [Button](#version-9x-button)
   - [Card](#version-9x-card)
   - [Chip](#version-9x-chip)
 
 <h2 id="version-9x-components">Components</h2>
 
+<h4 id="version-9x-button">Button</h4>
+
+- The `border-radius` of the `ios` and `md` button now defaults to `6px` and `999px` instead of `14px` and `4px`, respectively, in accordance with the iOS and Material Design 3 guidelines. To revert to the previous appearance, set the `shape` to `"soft"` for `md` and override the `--border-radius` CSS variable for `ios` to `14px`, or set it to a different value entirely.
+
 <h4 id="version-9x-card">Card</h4>
 
 - The `border-radius` of the `ios` and `md` card now defaults to `14px` and `12px` instead of `8px` and `4px`, respectively, in accordance with the iOS and Material Design 3 guidelines. To revert to the previous appearance, set the `shape` to `"soft"`, or override the `--border-radius` CSS variable to specify a different value.