Display custom fields possible values separator

It is currently hard for users to guess the separator to use for the
custom fields possible values.  Add a localized string to display
next to the field on the form

Fixes #17825
Increase size of custom field possible valus field
Revise string for 'separate lists by'
Merge pull request #533 from vboctor/Issue17825_custom_field_separator

Clarify separator that can be used to separate possible values

Fixes #17825