Add documentation note that Ada does not allow bitwise operations on signed integer types

[mborland]

We could always throw a category error when used or static_assert

[mborland]

We can also follow what's in the C++20 standard, basically:

[Note: Bitwise operations are performed as if on a two's-complement representation, where positive numbers have an infinite number of leading zeroes, and negative numbers have an infinite number of leading ones. — end note]