add phpdoc for iterators in unit tests #21403

Job	Run time
Compiler Tests	4m 27s
integration-tests / Check PHAR checksum	4s
extension-tests / Check PHAR checksum	5s
other-tests / Check PHAR checksum	4s
PHAR Prefix Diff	0s
Commit PHAR	0s
Checksum PHAR	0s
Download base SHA PHAR	0s
other-tests / Other Tests	-1s
integration-tests / Integration - ${{ matrix.repo }}	0s
integration-tests / Integration - ${{ matrix.name }}	0s
integration-tests / Integration - Update baselines	0s
extension-tests / Extension Tests	0s
	4m 39s

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