Update docs

[JamesWrigley]

• Update Documenter.jl
• Add/fix docstrings for a bunch of functions/types
• Expand reference page

Things that might need to be changed:

• AFAICT the docs are generated with Travis CI, but I don't think that's running anymore. Travis CI stopped free builds for public projects a while ago. Maybe the docs could be generated by appveyor? Or Github actions?

• Documenter.jl has a problem with recognizing inner constructors of structs, when I tried adding e.g. Socket(typ::Integer) I got:

  ┌ Warning: no docs found for 'Socket(typ::Integer)' in `@docs` block in src/reference.md:7-12
  │ ```@docs
  │ Socket
  │ Socket(typ::Integer)
  │ isopen
  │ close
  │ ```
  └ @ Documenter.Expanders ~/.julia/packages/Documenter/vylnb/src/Utilities/Utilities.jl:34

  Possibly related: Multiple constructors JuliaDocs/Documenter.jl#396
  I got around this by copying the constructor docstrings into the type docstrings and manually separating them with horizontal lines to approximate what Documenter.jl does with method overloads:
  

  I left the docstrings on the constructors too for the sake of clarity (though now they're duplicated so perhaps it would be better to just keep them in the type docstring).

Should fix #203.

[ViralBShah]

@JamesWrigley I'm seeing this now - would it be helpful for you to have commit access here?