[Store] Add documentation to InMemoryLoader class

Add description explaining that InMemoryLoader returns pre-loaded TextDocuments
from memory and is useful for testing or when documents are already available
as objects. Also update parameter type annotation from array<TextDocument> to
TextDocument[] for better readability.

Author: OskarStark

src/store/src/Document/Loader/InMemoryLoader.php

@@ -15,12 +15,15 @@
 use Symfony\AI\Store\Document\TextDocument;
 
 /**
+ * Loader that returns pre-loaded TextDocuments from memory.
+ * Useful for testing or when documents are already available as objects.
+ *
  * @author Oskar Stark <[email protected]>
  */
 final readonly class InMemoryLoader implements LoaderInterface
 {
     /**
-     * @param array<TextDocument> $documents
+     * @param TextDocument[] $documents
      */
     public function __construct(
         private array $documents = [],