Add documentation to config.php for share_folder

Signed-off-by: John Molakvoæ (skjnldsv) <skjnldsv@protonmail.com>

config/config.sample.php

@@ -1439,6 +1439,7 @@ $CONFIG = array(
 
 /**
  * Define a default folder for shared files and folders other than root.
+ * Changes to this value will only have effect on new shares.
  *
  * Defaults to ``/``
  */