Add docstrings!

[josephsdavid]

Week one of GSOD work! in service of #25 and #913

[ablaom]

There is no need to document any methods in MLJ interface code. The focus in JuliaAI/MLJ.jl#913 is on documenting the model types and that's where all documentation goes. While Julia is not OO, in this case we are kind of organising the documentation as if it were, and so a model docstring here looks a bit like an estimator docstring in sk-learn.

As you can see from the DecisionTree.jl example, if a model implements predict, then that is mentioned in the model docstring under "Operations". Similarly the output of fitted_params and report are also documented in the model doc-string.

All possible operations are listed here for example. The operations predict_mean and predict_mode and predict_median have fallbacks that call predict and are rarely overloaded by an interface. Every probabilistic classifier implements predict but gets predict_mode for free, so that is added to the docs, like in the DecisionTree.jl example.

[codecov-commenter]

Codecov Report

Merging #26 (c96ddf0) into dev (c496abe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev      #26   +/-   ##
=======================================
  Coverage   94.44%   94.44%           
=======================================
  Files           1        1           
  Lines          90       90           
=======================================
  Hits           85       85           
  Misses          5        5           

Impacted Files	Coverage Δ
src/MLJGLMInterface.jl	94.44% <ø> (ø)

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[ablaom]

I've finished reviewing LinearRegressor. Nice example.

[josephsdavid]

I've finished reviewing LinearRegressor. Nice example.

Woohoo! I should have linear classification and count regression knocked out soon, just working out these examples!

[ablaom]

@josephsdavid Very embarrassing, but I wrote "marginal" when I meant "conditional" in my suggestions for the intro docstrings. I've edited them accordingly. 😳

[ablaom]

It still looks like we need to check consistency of item line beginnings (lower case?) and endings (no period?)