Clarify in documentation that "slot-scope" is named "scope" before 2.5.0

[xak2000]

The guide about Scoped Slots says:

In 2.5.0 , slot-scope is no longer limited to the <template> element, but can instead be used on any element or component in the slot.

But it is not says, that until 2.5.0, slot-scope doesn't work at all, even inside <template> element. To make it work, It must be scope inside <template> element.

It will be good to mention in the same warning element in documentation, that slot-scope not works until 2.5.0 and scope must be used instead.

Something, like:

Until 2.5.0, slot-scope was named scope and was limited to the <template> element.
In 2.5.0 , slot-scope is no longer limited to the <template> element, but can instead be used on any element or component in the slot.

I spend several hours trying to understand why it says that this functionality should work from 2.1.0+, but it doesn't work until 2.5.0. And the reason: it works! Just slot-scope should be scope. :)

Related issue: #1223 but it fixed it only in API docs (not in the guide).
Related commit: vuejs/vue@dae173d

[phanan]

Let's move this to the PR.