Clarify separator that can be used to separate possible values

1. Show the separator to use to separate custom field possible values given that '|' is hard to guess.
2. Increase the width of the possible values field for readability of its content.

Fixes #17825
`'|'` looks like a something that should probably be defined in constant_inc.php, e.g. CUSTOM_FIELD_LIST_SEPARATOR, and use the new constant elsewhere (custom_field_api.php ?) as appropriate.