Improve doc block for Unicode::truncate()

This is still valid and is suitable for a first issue.

With the latest PHP 8.x I think we could avoid many of these phpDocs by just using the types inside the method signature.

jcorrao → changed the visibility of the branch 1965508-improve-doc-block to hidden.

I have gone through and reviewed the patch file from #1 and ported the changes to the relevant counterpart in the 11.x codebase.

The type declarations in the PHPDoc patch were present in 11.x already, however they were missing the `(optional)` declaration as described in the linked PHP Documentation Standards page: http://drupal.org/node/1354

MR !7990 is published and ready for review.

So re-ran the tests but getting many failures and not sure why with just comments.

Believe the issue has been fixed but just needs a rebase.