Replace markdown manual/reference documentation with actual docstrings + Documenter.jl

[awf]

Good documentation for btime:

help?> @btime
  @btime expression [other parameters...]

  Similar to the @time macro included with Julia, this executes an expression, printing the time it
  took to execute and the memory allocated before returning the value of the expression.

  Unlike @time, it uses the @benchmark macro, and accepts all of the same additional parameters as
  @benchmark. The printed time is the minimum elapsed time measured during the benchmark.

But bad documentation for the cross-referenced @benchmark:

help?> @benchmark
  No documentation found.

  BenchmarkTools.@benchmark is a macro.

  # 1 method for macro "@benchmark":
  [1] @benchmark(__source__::LineNumberNode, __module__::Module, args...) in BenchmarkTools at C:\Users\Andrew Fitzgibbon\.julia\packages\BenchmarkTools\mVOqg\src\execution.jl:209

I had to trace through the code to find

mutable struct Parameters
    seconds::Float64
    samples::Int
    evals::Int
    overhead::Float64
    gctrial::Bool
    gcsample::Bool
    time_tolerance::Float64
    memory_tolerance::Float64
end

And then guess from there.

I realize it may not be easy to autogenerate, but either way, help on @benchmark should do something useful.

[jrevels]

They're documented in the BenchmarkTools manual.

We should definitely update BenchmarkTools to use normal docstrings, Documenter.jl, etc.

[awf]

Thanks. In the short run, even a hyperlink to the manual would be considerably more ergonomic.