Minimize chance of accidental screwup by making limited display default

This was in the initial plan, but then I added an option to limit effects to admin users (those with 'administer @font-your-face' permission), and the module started to seem much less dangerous.

Now that I think of it, flipping the default value of that option would lower the probability of screwing up in case someone enables this on a live site without reading the documentation first.

Would that be enough?

This module was indeed conceived in the middle of the night, although no company or beverages other than coffee were involved. :)

I still think however it can be useful to discover font combinations that one would not have thought of otherwise.

The attached patch flips the default value of random_fonts_restricted variable and also strives to improve wording/documentation a bit.

Now that Edge Fonts is in (see #1794540: Add Edge fonts submodule), I'm preparing to roll this module out; therefore, #4 has been committed.

Feel free to reopen if you see something else that should or could be done to make it safer.