Add docstrings and update docs

[cjprybol]

Hi,

Thanks for developing this package! Just wanted to help update the documentation a bit, both to help clarify some things for myself but also hopefully for others.

• add convenience functions for kmer iterators to docs - currently online docs only show the docstring for the type
• add missing kmer type aliases to online docs
• change variable name in transformation and slicing function example to clarify that the object is a kmer rather than a sequence
• correct typo in ReverseGeneticCode @docs call for online docs
• add docstring and jldocstest for base.random(<:Kmer)
• add primary constructor function calls to docstrings for kmer iterator types

[cjprybol]

Thanks for the review and thoughtful feedback!

I wasn't able to get the StableRNG to work, seems like there are some missing functions that we'd need to implement to pull it off?

julia> rand(StableRNG(1), Kmer{DNAAlphabet{2}, 3})
ERROR: ArgumentError: Sampler for this object is not defined
Stacktrace:
 [1] Random.Sampler(#unused#::Type{StableRNGs.LehmerRNG}, sp::Random.SamplerType{DNAKmer{3, N} where N}, #unused#::Val{1})
   @ Random /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Random/src/Random.jl:145
 [2] Random.Sampler(rng::StableRNGs.LehmerRNG, x::Random.SamplerType{DNAKmer{3, N} where N}, r::Val{1})
   @ Random /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Random/src/Random.jl:139
 [3] rand(rng::StableRNGs.LehmerRNG, X::Random.SamplerType{DNAKmer{3, N} where N})
   @ Random /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Random/src/Random.jl:253
 [4] rand(rng::StableRNGs.LehmerRNG, ::Type{DNAKmer{3, N} where N})
   @ Random /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Random/src/Random.jl:256
 [5] top-level scope
   @ REPL[14]:1

Love the idea in general, and agree it would be worth adding to the doctest (and the regular tests), but adding more functions here (and possibly also in Base.Random?) feels like more than I'm ready to bite off with this pull request.

Instead, I simplified the example in the docstring by removing the random seed setting but also removed the jldoctest flag so it hopefully won't break tests across versions.

Let me know if you think that's a fair compromise if you have another idea for how to handle the RNG for this PR

[kescobo]

but that's not working for me at the moment...

Seems I misunderstood how this works - seed!(rng, 1) sets the seed for rng, not the global RNG. So new methods will indeed be needed.

Have you looked at the source of all of these test failures?

[cjprybol]

@kescobo hadn't looked at the test errors until you asked, but see this issue for what I found https://github.com/BioJulia/Kmers.jl/issues/33

The errors seem unrelated to these changes. Happy to iron them out in a different PR after making a decision on that issue!

[kescobo]

Agreed - You've got a green check on docs, and that's what matters for this PR. Thanks again for the contribution!