message: Clarify how witness works in revocation
diff: @@ -132,7 +132,8 @@ communicates two other pieces of vital information:
  lets the holder look up their private factor, which we could map to
  `a` in the simple equation from the accumulator background section
  at the top of the doc.
  -* The product of the other factors contributing to the accumulator.
  +* The product of the _other_ factors contributing to the accumulator (all
  + factors except the private one for this credential).
  This value is like `b * c * d` from the simple equation above, and
  is called a <font color="blue">witness</font>.
  