Add example workaround for conditional attributes when using unsafeStatic

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Trying to use `ifDefined` inside of `unsafeStatic` causes a type error from TS and a runtime error from JS: “Cannot convert a Symbol value to a string”. This wasn't immediately apparent to us and, in our case, our editors (Webstorm and VSCode) did not flag the type conversion error from TS at all when the invalid usage is wrapped inside of a `` element.

The workaround is to use a conditional to determine whether to render the attribute:
```typescript
import { html, unsafeStatic } from 'lit/static-html';

@property()
type?: string;

render() {
return html`
${unsafeStatic(`
some text
`)}
`
}
```

No documentation file or test is named. Start from the unsafeStatic documentation entry point and review the TypeScript example in this issue; done means documenting the conditional-attribute workaround and explaining the failing ifDefined usage and its errors.

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