Document CardTitle's cardTitle theme
Document CardTitle's cardTitle theme

The `cardTitle` theme for the root `CardTitle` component was not
documented.

I also made the wording for `title` and `subtitle` more consistent.

soyjavi/react-toolbox,showings/react-toolbox,jasonleibowitz/react-toolbox,soyjavi/react-toolbox,rubenmoya/react-toolbox,react-toolbox/react-toolbox,showings/react-toolbox,rubenmoya/react-toolbox,jasonleibowitz/react-toolbox,rubenmoya/react-toolbox,react-toolbox/react-toolbox,react-toolbox/react-toolbox