Add usecase bind + custom setter

[oleole39]

Document the case when using bind together with a custom setter.

That is what I found necessary for marl_ynh where the config file is of mixed format - a JS file containing an indexed array and an associative array (JSON). When only the second one is supported by the bind config panel parameter.

[oleole39]

And would someone be able to remember the reasoning behind the use of ynh_backup_if_checksum_is_different in the bind-related part of _ynh_app_config_apply_one ?

Would it make sense to follow the same logic in the setter-related part of the same helper (at least for the if block starting at L.80 as I don't really understand what is the one at L.83 aiming at) ?
Or to add ynh_store_file_checksum() there so that it would not be required to mention it in this PR ?