Clarify that Iterator->rewind() must be called before a foreach

From manual page: https://php.net/iterator.rewind

Many comments in the manual state that foreach calls the rewind method before iterating. But it always seems as if this is something that foreach just does, maybe for safety. But that's a misunderstanding. In a final comment in https://bugs.php.net/bug.php?id=63823, levim says very clearly that calling rewind before using an iterator is mandatory and trying to get values from an unrewound iterator is undefined behaviour. I can't find this clear statement in the manual. At least, it should be found in articles about the current() and next() methods (which could be many). This would have saved me some hours today...

Start with the Iterator::rewind() manual page and review the related current() and next() documentation mentioned in the issue. Clarify that rewind() must be called before reading an iterator and document the consequence of using an unrewound iterator, then verify the wording and documentation links in the PHP manual build.