Add docs for all exported functions

[inkydragon]

julia> using Combinatorics

julia> Docs.undocumented_names(Combinatorics)
14-element Vector{Symbol}:
 :Combinatorics
 :Partition
 :SkewDiagram
 :doublefactorial
 :fibonaccinum
 :hyperfactorial
 :jacobisymbol
 :legendresymbol
 :lucasnum
 :multifactorial
 :ncpartitions
 :primorial
 :stirlings1
 :stirlings2

• Combinatorics

factorials.jl

• doublefactorial
• hyperfactorial
• multifactorial
• primorial

numbers.jl

• fibonaccinum
• jacobisymbol
• legendresymbol
• lucasnum
• stirlings1
• stirlings2

partitions.jl

doc: add more examples and test for partitions #181

• ncpartitions

youngdiagrams.jl

• Partition
• SkewDiagram