doc: add a note on typing Snippets in $props

[Lofty-Brambles]

Describe the problem

When trying to type $props, a common requirement tends to be providing a type to children. To a complete svelte newbie, it might be helpful to let them know that it is a snippet. Would it be pertinent to have a note linking to the typing snippets section inside the documentation for $props for that? Somewhere around/below the type safety section?

Describe the proposed solution

Something along the lines of:

[!NOTE] It is quite common to provide a type for children. children is typed as a Snippet, an section on providing types for Snippets is provided here.

Importance

nice to have

[Lofty-Brambles]

I'm open to dropping a small PR if this is okay-ed.