[docs/task] Consider adding note about typing args array with multiple types

[augustjk]

Consider this example
https://stackblitz.com/edit/vitejs-vite-bjrnbd?file=src%2Fmy-element.ts

The args function returns an array with 3 items of different types. Without adding as const, the inferred type of the args in the task function is an array where each item is a union of all possible values.

At the very least, we should include this for the TS code sample, and make a note of why it's needed. https://lit.dev/docs/data/task/#the-task-function

[augustjk]

Fixed by #1329