Cautions around @state properties

@arthurevans is already working on this.

Since Apr 26, 2022.

Language on the decorators page is a little bit misleading. https://lit.dev/docs/api/decorators/#state
- state props can be used in the component's own template, where any compiler prop renaming should be handled correctly.
- But references from _outside_ template systems or imperative JS would break.

Also note that if you need to access the property by a string name--for example, checking:

```
changedProperties.has('statePropertyName');
```

Will fail if you're using a compiler with prop renaming. You can work around this using the compiler's APIs. For example, in closure you'd use `objectProperty('statePropertyName', this)` ...

You also can't refer to the state property from the outside, like:

```

```

This relies on the string name of the property being unchanged. You just shouldn't do this—the state property should be internal to the component.