Rewrite example on for page

[kamil-tekiela]

I saw this example on for page, which appeared to me as misleading. I kept thinking about it all day and I decided that it needs to change. count was used as an example of an expensive operation, which it is not when used with arrays. It can be expensive on objects. What's more important, though, is that the example failed to explain why it's a bad idea to do it this way (it was much worse in the original version years ago). So the example needed modernizing and I took the inspiration from the user note on that page, which is now folded into the new example.

User note: https://www.php.net/manual/en/control-structures.for.php#13463
My explanation on Stack Overflow: https://stackoverflow.com/a/79991793/1839439