docs(material/radio): update MDC migration guide

Updates the MDC migration guide to reflect the changes to the radio button.

Author: crisbeto

guides/v15-mdc-migration.md

@@ -455,9 +455,22 @@ TODO(mmalerba): link to density docs once they exist.
 
 ### Radio
 
-* Radio-button labels are no longer `width: 100%`. This helps prevent users from accidentally
+* Radio button labels are no longer `width: 100%`. This helps prevent users from accidentally
   selecting a radio when clicking on whitespace that appears inert on the page.
 
+* Labels are smaller and further away from the radio button to align with the Material Design spec.
+
+* Accessibility: The touch target is much larger than the radio button; to match the legacy size,
+  you can provide a `-5` density to the radio button's theme mixin:
+
+```scss
+@use '@angular/material' as mat;
+
+@include mat.radio-theme(
+  map-merge($theme, (density: -5))
+);
+```
+
 ### Select
 
 * `MatSelect` no longer aligns the selected option in the dropdown menu with the trigger text.