Improve list of functions in the docs

[fredRos]

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.

[ararslan]

That table was just copied from the documentation as it existed in Base before it was moved to this package. It should probably just be removed.

[musm]

Yes it should be either (1) deleted or (2) updated.