Add documentation to extend JSX typing

[trusktr]

Re-opening of

• Add documentation to extend JSX typing core#2393

because it was never completed (but the stale bot marked it as completed, which makes me want to 🤖💥🔫, especially bots that immediately close and lock issues after marking them stale without leaving time for replies to confirm non-stale).

The docs do not mention how to extend JSX types for custom elements not written in Stencil (f.e. from 3rd-party libraries).

[christian-bromann]

Thanks for raising the issue. Any contributions would be much appreciated!