Changed some of the wording in units docs to clarify what "unit formats" is about

This comes from some discussion with @mdboom and @alexrudy  in #1462. What do you think about this change?

My thought is that if a user is thinking "how do I get my unit to look nice in a string", they won't necessarily know to look for something called "unit format", so that motivates the title change.  The rest is basically just small tweaks.

I'll tag this 0.3.1 so as not to delay anything, but it certainly can go in 0.3 if it's fine with everyone.