Document media prefixing naming conventions

[timkelty]

While media query prefixing exists in current iterations of utilities (e.g. https://github.com/suitcss/utils-size), there is nothing in the naming convention docs describing this.

Furthermore, it should be stated whether or not these should be used just with utilities, or with components as well. At one point it seemed this was kosher, but it seems that changed. I would argue to still have this as part or the naming convention for both utilities and components.

The convention also seems to have changed from .md-u-size1of2 to .u-md-size1of2 at some point as well.

I find media prefixes on components to be very valuable and would love to see that part of the naming convention brought back or redefined. More discussion on that topic here.

[necolas]

I've added mention of the utility naming convention in the docs.

For components, you can use media queries directly in your component CSS instead. And in general, I'm not really sure that viewport-width should be the concern of UI components. Certain frameworks will allow you to efficiently re-render the component HTML, and I think swapping out HTML is often simpler and more robust.