[css-cascade-5] Clarify the scope of layers

[cdoublev]

I.e. the scope is origin and context, as noted before the updated text:

Declarations within each origin and context can be explicitly assigned to a cascade layer.

In my opinion, an example would help to better capture what this means:

Given the following markup:

<style>
  @layer one { p::after { content: 'one wins' } }
  @layer two { p::after { content: 'two wins' } }
</style>
<p>Result: </p>
<div>
  <template>
    <style>
      @layer two { p::after { content: 'two wins' } }
      @layer one { p::after { content: 'one wins' } }
    </style>
    <p>Result: </p>
  </template>
</div>

The @layer rules in the light tree do not affect the @layer rules declared in the shadow tree.

In the light tree, the result is two wins. In the shadow tree, it is one wins.

[fantasai]

Makes sense. Suggest linking the terms. I'm unsure whether to use the long form or short form for the terms, though.