__quantum__rt__bigint_bitnot: Consider documenting better

[kuzminrobin]

In the name of __quantum__rt__bigint_bitnot the ending bitnot makes an impression that it is the bitwise negation. But the documentation says "bitwise complement", from which it is unclear whether it is a
Two's complement
or Ones' complement (equivalent to bitwise negation).

Consider documenting either as "bitwise negation" or "ones' complement".
If it is two's complement, then consider removing the word "bitwise" (I believe the two's complement is not a bitwise concept).