Add note about prefix with gridfs api

Author: GromNaN

docs/filesystems.txt

@@ -151,3 +151,9 @@ if you need to work with revisions, as shown in the following code:
 
    // Download the last but one version of a file
    $bucket->openDownloadStreamByName('hello.txt', ['revision' => -2])
+
+.. note::
+
+   The Filesystem component handles the path prefix, and removes any leading
+   directory separator ``/``. If you use the GridFS API directly, you have to
+   manage it by yourself.