Issue #2791211 by cilefen: Cache::getBins should explain the returned data...

Issue #2791211 by cilefen: Cache::getBins should explain the returned data types implement CacheBackendInterface

core/lib/Drupal/Core/Cache/Cache.php

@@ -148,7 +148,7 @@ public static function invalidateTags(array $tags) {
   /**
    * Gets all cache bin services.
    *
-   * @return array
+   * @return \Drupal\Core\Cache\CacheBackendInterface[]
    *   An array of cache backend objects keyed by cache bins.
    */
   public static function getBins() {