Improve utility classes and documentation for typography

[hartsick]

When Symonne and I were working with Honeycrisp, we noticed that there was one text utility class grouping (text-25 and text-25-small) but not the others you'd expect from our spacing variables (mainlytext-35 and text-60). I think this naming convention also differs and maybe should be text--25 instead, though not totally sure about that.

I'd like to propose expanding the typography utility classes to have consistent class availability for all allowable spacings, and expanding the documentation to tell folks how to use them (currently missing).