Clearer wording of allow_delete template behavior

[brooksvb]

The allow_delete option has one consequence: if an item of a collection isn't sent on submission, the related data is removed from the collection on the server. The solution is to remove the form element from the DOM.

The solution is to remove the form element from the DOM is a confusing sentence. Saying "the solution" implies that there is some sort of problem with enabling allow_delete that needs to be solved. I found it very confusing what the problem was, and reread the section multiple times to figure out what it was supposed to be.

I finally understood that what it's trying to say is that if you want the intended behavior to work, (not problematic, because you are enabling the behavior intentionally), you have to remove the collection item from the submitted data. In terms of submitting the form through a webpage, that means removing the DOM element.

Also removed has one consequence from the first sentence of the paragraph, because calling it a consequence, again, implies that it's a behavior you don't really want, despite the fact you're enabling it intentionally.

Overall, I think these small changes would prevent some unnecessary confusion like what I experienced.

[brooksvb]

This change is relevant for 2.7 and up

[javiereguiluz]

@woofiewilly this is a perfect first-time contribution to Symfony Docs. You explained the problem in detail and provided a simple solution that improves the docs ... and you used the right branch (2.7), which is usually the tricky part for newcomers. Thanks!