Add docstring for sigma and other objects.

[codetalker7]

This is in conjunction with PR #12. Currently the PR does not contain any docstring for the sigma getter function in src/frequentist/getter.jl. This function is not in StatsAPI.jl either, we'll have to add our own docstring to it, describing what it does.

[codetalker7]

While we're on the subject, a lot of improvements in the documentation can be made. Roughly the following comes to mind as of now.

1. Add documentation for priors (currently we don't have any docstrings for those).
2. Add more documentation describing the models.
3. In relation to 2), add more documentation mathematically describing the bayesian models, eg. what are the prior distributions, how is inference done etc.

Currently the docstrings for everything are quite general and don't explain the specifics of the models.

[sourish-cmi]

The Bayesian code with Turing.jl is a great extent, self-explanatory. We can directly take the code-piece and make it part of the doc

[codetalker7]

I see. On a side note, it will be better to write the models in LaTeX instead of Julia code, no?