Create MLJ standard docstrings for all Nearest Neighbor models

[OkonSamuel]

Addresses #51

[codecov-commenter]

Codecov Report

Merging #53 (f83ca08) into dev (ea14cce) will decrease coverage by 0.24%.
The diff coverage is 50.00%.

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@@            Coverage Diff             @@
##              dev      #53      +/-   ##
==========================================
- Coverage   93.16%   92.92%   -0.24%     
==========================================
  Files           4        3       -1     
  Lines         395      396       +1     
==========================================
  Hits          368      368              
- Misses         27       28       +1     

Impacted Files	Coverage Δ
src/models.jl	91.58% <50.00%> (-0.42%)	⬇️

... and 1 file with indirect coverage changes

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[ablaom]

Great to have this @OkonSamuel .

Apart from the comments above:

• We need to move the docstrings so that they occur after the metadata declaration because the docstring header interpolator is not working:

A model type for constructing a multitarget knn regressor, based on unknown.jl (unknown), and implementing
  the MLJ model interface.

• I suggest we declare human_name traits, eg
  human_name = "K-nearest neighbor classifier" to make the doc header read better.

[ablaom]

Thanks again for this.