[css-scoping] Update ::slotted (section 3.2.3)

::content is gone, and in Shadow DOM v1 world, ::slotted() will replace it.

The discussion happened here:
https://github.com/w3c/webcomponents/issues/331

Now it seems the spec update is halfway done.
https://drafts.csswg.org/css-scoping-1/#content-combinator

* the hash anchor should be #slotted-pseudo :)
* It is "::slotted()" and takes one compound selector as an argument, and
it cannot be omitted
* ISSUE 7 is already resolved (only compound selector is allowed, no
combinator)
* EXAMPLE 7 should use slot="..." and <slot name="..."> instead of <content
select="...">
* The second paragraph after EXAMPLE 7's HTML example is no longer necessary

Tab, could you update the text?
-- 
Takayoshi Kochi