Improve docblocks

Closes #3510810

importmap.module

 <?php
 
+/**
+ * @file
+ * Hooks for the importmap module.
+ */
+
 use Drupal\Component\Serialization\Json;
 use Drupal\Core\Render\BubbleableMetadata;
 use Drupal\Core\Url;
 
 /**
- * @return array
- * @see          \Drupal\libraries_ui\LibrariesUiService::getAllLibraries()
+ * Get a list of libraries declared by core, modules and themes.
+ *
+ * @return mixed[][]
+ *   List of libraries.
+ *
+ * @see \Drupal\libraries_ui\LibrariesUiService::getAllLibraries()
  * @noinspection PhpUndefinedNamespaceInspection
- * @phpstan-return mixed[][]
  */
 function _importmap_get_libraries(): array {
   /** @var \Drupal\Core\Asset\LibraryDiscoveryInterface $libraryDiscovery */