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
This is beyond the scope of this change.  I've opened a code cleanup bug for this that explains the appropriate fix for this.  http://www.mantisbt.org/bugs/view.php?id=17828