add $slots property to API docs

[kazupon]

I realized that $slots property is nothing in guide/api/index.md, when I found that dead link in render function.

[kazupon]

@chrisvfritz thanks!

[chrisvfritz]

@kazupon Thanks for catching it!

[jbruni]

Great! Just stumbled upon it. Cool that it has already been resolved.

[jbruni]

Hmmm... the explanation and code samples seem confusing to me.

[kazupon]

sorry, my poor english ... 😞

[LinusBorg]

@jbruni Have you read the sections linked under the example?

The content of $slots can only be understood (and be of any use zo you) to you when you use render functions - in which case you will have to understand what vNodes are, which is explained in the linked guide sections ..

[jbruni]

@LinusBorg - Yes, I do understand the context, in ttheory and practice. The text could be much simpler, and the code (only perceived now) has a little error (reference to "slot2" and "slot1"), and in my opinion the code does not help clarifying the contents. I'll see if I can quickly write and send a suggestion as PR.