Get documentation on "StyleHints"?

I’ve done some searching but I can’t seem to find any documentation on “StyleHints”. I’m using 11.3 on OS X High Sierra and there doesn’t seem to be any built-in documentation. A search on this site also didn’t turn up much. Is there any formal documentation? Has anyone posted a tutorial somewhere?

At least there is no direct entry on the Wolfram Reference Pages. Are you sure it’s built-in?