Update doc block to indicate absolute path requirement.

Fixes #12567

src/Filesystem/File.php

@@ -574,7 +574,7 @@ class File
     /**
      * Copy the File to $dest
      *
-     * @param string $dest Destination for the copy
+     * @param string $dest Absolute path to copy the file to.
      * @param bool $overwrite Overwrite $dest if exists
      * @return bool Success
      */