Fix-up and clean-up docs for int.bit_length().

Fix-up and clean-up docs for int.bit_length().

* Replace dramatic footnote with in-line comment about possible round-off errors in logarithms of large numbers.
* Add comments to the pure python code equivalent.
* replace floor() with int() in the mathematical equivalent so the type is correct (should be an int, not a float).
* add abs() to the mathematical equivalent so that it matches the previous line that it is supposed to be equivalent to.
* make one combined example with a negative input.