[reactive-element] query decorator API doc examples don't type-check

Nobody has claimed this yet.

### Which package(s) are affected?

Lit Core (lit / lit-html / lit-element / reactive-element)

### Description

The [query](https://lit.dev/docs/api/decorators/#query) decorator API docs show TS code that does not type-check (`Property has no initializer and is not definitely assigned in the constructor`):

```ts
@query('#first')
first: HTMLDivElement;
```

The most correct is to show it as nullable, since if you query before the first `update()` or if the template contains conditionals, it may return null:

```ts
@query('#first')
first: HTMLDivElement | null = null;
```

OTOH, most users may opt for typing it as non-nullable since in practice that'll often be safe:

```ts
@query('#first')
first: HTMLDivElement!;
```

The API docs should be updated to reflect one of the two patterns. The former is safer, the latter is more terse.

### Reproduction

From API docs:

```ts
class MyElement {
@query('#first')
first: HTMLDivElement;

render() {
return html`

`;
}
}
```

### Workaround

Use a pattern above

### Is this a regression?

No or unsure. This never worked, or I haven't tried before.

### Affected versions

Latest

### Browser/OS/Node environment

Any

Start at the linked query decorator API documentation and inspect the TypeScript example showing the uninitialized `first` property. Update the example to use one of the type-safe nullable or non-nullable patterns proposed in the issue, then verify that the displayed code type-checks.

Written by the indexing model from the issue text.