Improve list of functions in the docs

The list of functions at http://juliamath.github.io/SpecialFunctions.jl/latest/ doesn't cover all the implemented functions. For example, `polygamma` is not documented although it is available. Also the ordering of functions in the table seems incomprehensible.
The list at http://juliamath.github.io/SpecialFunctions.jl/latest/special.html has all the functions as it seems autogenerated from the source code. How about just removing the hand-maintained table? Following the DRY principle, the autogenerated list is much better in terms of maintenance cost.