docs(app): add setFocus to the documentation (#29916)

Issue number: resolves #29830

---------

## What is the current behavior?
The `setFocus` method on `ion-app` is marked internal.

## What is the new behavior?
Document the `setFocus` method as a way for developers to
programmatically focus elements.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

## Other information

The method isn’t new, it was just marked as internal, which prevented it
from being documented. I can mark this as a `feat` though if anyone
thinks it should be.

Related documentation PR:
https://github.com/ionic-team/ionic-docs/pull/3842

core/src/components/app/app.tsx

@@ -61,7 +61,6 @@ export class App implements ComponentInterface {
   }
 
   /**
-   * @internal
    * Used to set focus on an element that uses `ion-focusable`.
    * Do not use this if focusing the element as a result of a keyboard
    * event as the focus utility should handle this for us. This method