Improve doc on hiding fields

Need to improve the document for hiding fields

Currently, the description appears to say the same thing twice and fails to document the vital difference between hiding the field and removing it from all screens.

Hiding a field from the field configuration page will make JIRA think that the field does not exist for the associated project, hence it is not shown on any associated screens and the fields default value is not set.

Whereas if you remove the field from all the associated screens, it will not show the field on any of the associated screens but the fields default value is set as JIRA knows about it.

I think having some kind of matrix will help explain it better:
Eg. The following table describes the behaviour of a field depending on whether the field is

• Shown/Hidden on the field configuration
• Required/Optional on the field configuration
• On/Off an associated screen

  Field Visibility / Requirement	Required	Optional
  Show & on screen	Field value is required and must be selected/entered	Field value is optional - can have no input selected/entered
  Show & off screen	Uses default value if it has one, otherwise causes 'field value required' error	Uses default value if it has one, otherwise stores no value
  Hide	Field is ignored and stores no value	FIeld is Ignored and stores no value