ICU-22877 revised "Region code" (formerly "Country code") section

[cjchapman]

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22877
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [x] Issue accepted (done by Technical Committee after discussion)
• ~~[ ] Tests included, if applicable~~ (not applicable)
• [x] API docs and/or User Guide docs changed or added, if applicable