ICU-22722 toTitle does not use UTR 21

[markusicu]

Fix API docs that claim that titlecasing functions use Unicode Technical Report #‌21 for the titlecase break iterator.
That was the case only for Unicode 3.2.
Use the wording from the Java API docs instead, which generically refers to the Unicode case mapping algorithms so that we need not adjust the docs every time this changes.

(Titlecasing has been using the word break iterator since Unicode 4.)

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22722
• 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

[richgillam]

Looks fine to me.