Issue #1965508: Improve documentation for Unicode::truncate()

Adds the "(optional)" to parameter descriptions in the PHPDoc for \Drupal\Component\Utility\Unicode::truncate() as per http://drupal.org/node/1354

Re-rolls an older 8.x patch for /core/includes/unicode.inc to 11.x, in which this ticket is currently designated for.

Closes #1965508

added 765 commits

• eb4c9596...acd9e7c8 - 763 commits from branch project:11.x
• 74aa3137 - Issue #1965508: Improve documentation for Unicode::truncate()
• 64b39d29 - Merge branch '1965508-improve-doc-block' of...

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added 634 commits

• 64b39d29...711e9ad4 - 633 commits from branch project:11.x
• 13d1ade9 - Issue #1965508: Improve documentation for Unicode::truncate()

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