#description_display doesn't work with multiple value tableselect field

The usual fixes for this type of problem are done by adding something like this to a template file:

{% if title %}
  <h4{{ title_attributes.addClass(title_classes) }}>{{ title }}</h4>
{% endif %}
{% if description_display == 'before' and description %}
  <div{{ description_attributes.addClass(description_classes) }}>
    {{ description }}
  </div>
{% endif %}

But that won't work here because the markup for the description text needs to be added inside a th in the table. For this, we'd need to update template_preprocess_field_multiple_value_form() in core's theme.incand edit the code that creates the table header.

The diff would look something like this:

diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index fbf78cf66b..ddb97d05a5 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1603,14 +1603,38 @@ function template_preprocess_field_multiple_value_form(&$variables) {
       $header_attributes['class'][] = 'js-form-required';
       $header_attributes['class'][] = 'form-required';
     }
+
+    $header_label = [
+      'label' => [
+        '#type' => 'html_tag',
+        '#tag' => 'h4',
+        '#value' => $element['#title'],
+        '#attributes' => $header_attributes,
+      ],
+    ];
+
+    $description_id = NULL;
+    if (!empty($element['#description'])) {
+      $description_id = $element['#attributes']['aria-describedby'];
+      $description_attributes['id'] = $description_id;
+      $variables['description']['attributes'] = new Attribute($description_attributes);
+      if ($element['#description_display'] == 'before') {
+        $variables['description']['attributes']->addClass('description');
+        $header_label['description'] = [
+          '#type' => 'html_tag',
+          '#tag' => 'div',
+          '#value' => $element['#description'],
+          '#attributes' => $variables['description']['attributes'],
+        ];
+      }
+      else {
+        $variables['description']['content'] = $element['#description'];
+      }
+    }
+
     $header = [
       [
-        'data' => [
-          '#type' => 'html_tag',
-          '#tag' => 'h4',
-          '#value' => $element['#title'],
-          '#attributes' => $header_attributes,
-        ],
+        'data' => $header_label,
         'colspan' => 2,
         'class' => ['field-label'],
       ],
@@ -1678,14 +1702,9 @@ function template_preprocess_field_multiple_value_form(&$variables) {
       ],
     ];

-    if (!empty($element['#description'])) {
-      $description_id = $element['#attributes']['aria-describedby'];
-      $description_attributes['id'] = $description_id;
-      $variables['description']['attributes'] = new Attribute($description_attributes);
-      $variables['description']['content'] = $element['#description'];
-
+    if ($description_id) {
       // Add the description's id to the table aria attributes.
-      $variables['table']['#attributes']['aria-describedby'] = $element['#attributes']['aria-describedby'];
+      $variables['table']['#attributes']['aria-describedby'] = $description_id;
     }
   }
   else {

But, in order for that to look right, CSS would be need to be updated, too. Without that, the description text would end up being bold, too, because it's in a table header. Something like this would need to be added to themes:

.field-multiple-table th.field-label div.description {
  font-weight: normal;
}

Also, Claro and other themes generally add their own class to the description text to format it so it's smaller. Each theme that needed that would have to implement a hook like this:

/**
 * Implements hook_preprocess_HOOK().
 */
function claro_preprocess_field_multiple_value_form(&$variables) {
  if (!empty($variables['table']['#header'][0]['data']['description'])) {
    $variables['table']['#header'][0]['data']['description']['#attributes']['class'][] = 'form-item__description';
  }
}

I'm also not quite sure where $element['#description_display'] = 'before' could be set since template_preprocess_field_multiple_value_form() runs so early.

Does this seem like the right path to go down? If so, I can created a patch / MR that does this.

There's also the Top Description → module that I created that handles this in its own implementation of hook_preprocess_field_multiple_value_form().