ICU-21935 DisplayOptions cleanup

[markusicu]

Cleanup after PR #2061 -- sorry for not reviewing that on time.
Indentation, @params, don't qualify C enum constants, other C++ style & helpers, ...

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-21935
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable