Remove incorrect property in docstring (re #9532)

The property

  countLeadingZeros . negate = const 0

doesn't generally hold and it's not such a useful property to state, as
it simply follows from "sign-bit == most-significant-bit" for FiniteBits
types which use twos-complement representation for negative values, and
even then it breaks down for 0...

TLDR, remove thinko from documentation of `countLeadingZeros`