fix doc block of OC\Preview\Provider::getThumbnail()

lib/private/preview/provider.php

@@ -11,9 +11,15 @@ abstract class Provider {
 	abstract public function getMimeType();
 
 	/**
-	 * search for $query
-	 * @param string $query
-	 * @return
+	 * get thumbnail for file at path $path
+	 * @param string $path Path of file
+	 * @param int $maxX The maximum X size of the thumbnail. It can be smaller depending on the shape of the image
+	 * @param int $maxY The maximum Y size of the thumbnail. It can be smaller depending on the shape of the image
+	 * @param bool $scalingUp Disable/Enable upscaling of previews
+	 * @param object $fileview fileview object of user folder
+	 * @return mixed
+	 * 		false if no preview was generated
+	 *		OC_Image object of the preview
 	 */
 	abstract public function getThumbnail($path, $maxX, $maxY, $scalingup, $fileview);
 }