Docs: Raise importance of global vs local transitions

[midzelis]

I got caught up wondering why my transition wasn't working - and it was because I needed a global transition. As currently written, the sentence:

A transition is triggered by an element entering or leaving the DOM as a result of a state change.

Makes it sound like transitions are always triggered by elements entering/leaving the DOM - however, that is only true for global transitions, which are not the default. I took a stab at clarifying this, and raising the importance of this by moving up the local/global section to the first paragraph. I created a dedicated section for 'Global Transitions' which contains the example, and I added an explanation of how to actually specify |global

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