Explain custom select styling

Created by: Josh68

I may be alone here, but I went down a rabbit hole trying to find documentation for styling "custom" selects, finally realizing that they're only custom in terms of styling the input itself, and not a library that rebuilds the menu of options/optgroups as style-able elements.

The only documentation about the various custom form controls is

For even more customization and cross browser consistency, use our completely custom form elements to replace the browser defaults. They’re built on top of semantic and accessible markup, so they’re solid replacements for any default form control.

Unless additional documentation calls out the fact that custom selects are just native elements (and that options cannot be styled), I think

completely custom form elements

is misleading.

If you agree, I could do a documentation edit for review and submit a PR.

Thanks