[docs] [standalone lit-html] calling render on a subtree is not recommended

Nobody has claimed this yet.

Expand https://lit.dev/docs/libraries/standalone-templates/ to document that calling `render` on an element that is part of a lit managed subtree is not recommended.

Came up in discussion of https://github.com/lit/lit/issues/2189

Repro, what is the final rendered DOM?

```js
import {html, render} from 'lit';
const ui = (v1, v2) => html`

`;
render(ui('one', 'two'), document.body);

render(html`

Some

more

DOM

`,
document.querySelector('#binding-1')!);

render(ui('three', 'four'), document.body);
```

Start with the standalone templates page at lit.dev/docs/libraries/standalone-templates/ and run the supplied render example to inspect its final DOM. Update that documentation to explain why rendering into an element within a Lit-managed subtree is not recommended, including the behavior shown by the repro.

Written by the indexing model from the issue text.