Add explanatory comment to details box-sizing fix

tildes/scss/_base.scss

@@ -59,6 +59,8 @@ code {
   line-height: 1rem;
 }
 
+// Fixes a Chrome bug that seems to treat children of <details> as having
+// box-sizing: content-box. See Known issues: https://caniuse.com/#feat=details
 details * {
   box-sizing: border-box;
 }