ICU-22952 Make configure --help show --enable-* / --disable-* consistently

[roubert]

It should print just how to toggle the default. Following this shema, it is not necessary to include default=yes/no in the output.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22952
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• 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. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

ALLOW_MANY_COMMITS=true

[srl295]

Much better