Update guide for the counter variable when rendering with the as: option

[lucasprag]

Summary

This PR adds a note to the guide to make sure people understand that when rendering a collection using partials like <%= render partial: "product", collection: @products, as: :item %>, the counter variable name will not be product_counter, but item_counter because of the as: option.

It should close #34199

This is my first PR to rails, I'm just trying to contribute, thanks.

[rails-bot]

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[y-yagi]

@lucasprag Thanks for your contribution!