Update the RichText documentation with an onSetup example

[notnownikki]

Description

Added some documentation about the onSetup prop, and an example of how to use it to set the focus on a newly created editor.

[youknowriad]

Thanks for the addition, left a small comment.

[youknowriad]

Is this common? I personally don't think so, I'd just use a more generic example here Something like:

function onSetup( editor ) {
  editor.on( 'focus', () => console.log( 'The editor has been focused' ) );
}

[notnownikki]

I don't know how common it will be... but if you're dealing with lists of items that have RichText components in them and you want to focus new ones, it is a gotcha. Perhaps that the focus event example in there, because that's an example more people are likely to use, and the note about focusing the editor afterwards?

[youknowriad]

I don't think personally either of these two examples is common because in the last versions of Gutenberg we are trying to do this automatically (handle focus). I know it's not solved entirely for all use-cases. My concern is that people would assume the onSetup is only useful to handle focus (in both examples)

[aduth]

This prop has since been marked as unstable as intentionally undocumented.

See: #9106