ICU-22397 Better UserGuide explanation of date pattern char Y

[pedberg-icu]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22397
• 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

Better UserGuide explanation of date pattern char Y. The UserGuide already refers people to the more detailed Date Field Symbol table in the CLDR docs, but people seem to not go there for some pattern chars, including this one.

[richgillam]

Belated LGTM.