comment-130+131+132

- Change "Registers" to "Register" in bullet 1, same for "Defines" to
  "Define" in remainder
- Add references.
- There is no ambiguity at the level the signature is calculated, but
  some text is added to remind the designer of Usages that they need to
  say how charcaters are converted to binary storage.
- s/hashed/used/