Documented the support of iterators in write() and writeln()#9776

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javiereguiluz wants to merge 2 commits intosymfony:4.1from

javiereguiluz:fix_9634

Commits on May 15, 2018

Documented the support of iterators in write() and writeln()
javiereguiluz
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Commits on May 21, 2018

Added a link to PHP's iterator article
javiereguiluz
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