Add additional detail to Button iconOnly prop

[ylanluu-ms]

I've been using fluentui-react-native/Button and needed it to become a square. I would set the width in styling to attempt this, but noticed that the width would not go below a certain set width. I browsed through documentation but could not find anything specifying something was hard setting the width. The solution ended up being using the iconOnly prop, which removes the hard set width. It would be helpful to add this detail in the documentation to prevent future users from running into the same confusion as me.