Document string-formatted arguments

Some arguments in Hector are provided as a `String`, although they really represent multiple things. The encoding for this is currently undocumented and require a look in the source code to find out. There are many many more things in the source code that have no documentation at all, consider this my small contribution in fixing this :).

The reference included for the comparator type alias is the best I could find in the DataStax documentation, though it might not be the one it's actually based on.