ICU-22404 Improve documentation of segmentation rules

[eggrobin]

See comments on #2492 and email thread with Andy titled genbrk.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22404
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• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[aheninger]

There is an out-of-date comment at the top of genbrk.cpp that should also be updated, concerning BOMs and rules encoding. Lines 23-27

Historically, the rule files have been UTF-8. Code for genbrk to ignore a BOM if present dates to the early days of using SVN for ICU version control - on Windows, BOMs are needed on local rules files, and unless svn clients were configured just so, the BOMs had a tendency to migrate into the repository's files too. Meanwhile, on other platforms, some text editors silently strip BOMs if they see them. The net result was BOMs were hit or miss. Maybe they're there, maybe not, and genbrk just had to deal with it.

[jira-pull-request-webhook]

Hooray! The files in the branch are the same across the force-push. 😃

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