Add comments about deleting this code in future

If the :has pseudo-class becomes available in the
browsers we support in future, the code this
branch/pull request adds will be redundant as its
behaviour will be possible only using CSS.

This adds comments to note this to the parts of
the code you'd need to remove.

app/assets/javascripts/main.js

@@ -25,6 +25,10 @@ $(() => $('.govuk-header__container').on('click', function() {
   $(this).css('border-color', '#005ea5');
 }));
 
+// Applies our expanded focus style to the siblings of links when that link is wrapped in a heading.
+//
+// This will be possible in CSS in the future, using the :has pseudo-class. When :has is available
+// in the browsers we support, this code can be replaced with a CSS-only solution.
 $('.js-mark-focus-on-parent').on('focus blur', '*', e => {
   $target = $(e.target);
   if (e.type === 'focusin') {