Modified the relative-units use-case

In order to resolve https://github.com/ResponsiveImagesCG/ri-usecases/issues/32, I've modified the responsive units section.

I have not fully understood the problem up until yesterday (Thanks [Marc](https://twitter.com/MarcDrummond)). It is not an issue of developer convenience, but of catering for both the 1em==16px users (most of the users) and the 1em==???px users (users who have set a different default font size, or that their device did it for them).

In these cases, without em based units for art-direction, developers will have to choose between PX based MQs, resulting in text overflows, or EM based MQs, resulting in the wrong art-directed images in some cases. (possibly breaking layout or getting distorted)

We still probably need to add a figure that describes this issue.