Document higher order components

[kylecordes]

Recently, a Svelte user asked if svelte has "higher-order components", like React (and many others). Rich pointed to the following example (which he tweeted previously) ofof doing this.

https://svelte.technology/repl?version=2.10.1&gist=b42e56c57e390b85e7c3c16349205cc6

Therefore, a documentation request: document higher order components in the official Guide.

Although it is a relatively advanced idea (thus some risk of confusing a new reader), nonetheless it would prevent someone comparing alternative technologies, from erroneously concluding that Svelte lacks this capability.

[ekhaled]

Related full convo #1648

[PaulMaly]

@kylecordes Maybe we should move this issue to docs repo?

[kylecordes]

@PaulMaly Yes... I hadn't noticed doc issues were over there instead. Will check for dups, and reopen there.

[kylecordes]

moved to sveltejs/v2.svelte.dev#353