Clean up and standardize function help blurbs

Not all the functions in `cryptanalib` have help blurbs. Let's document all the functions and their expected return values and whatnot like so:

```
"""
Get integer nth roots.

:param x: The number to get the integer root of.
:param n: The root we want (e.g. 3 for cube root).
:return: The nth root of x rounded to an integer.
:rtype: long
"""
```