Fix misuse of *usage* throughout Reference prose

*Usage* means a customary pattern of use (as in "regional usage
varies") or a rate or quantity of use (as in "disk usage").  It
doesn't mean *use*.  Whenever *use* works, *usage* shouldn't appear.

Let's fix each incorrect use of *usage* -- i.e., adopt better *usage*
of *usage* -- throughout the prose of the Reference.