XML documentation for GAM binary classifier trainer.

[codemzs]

towards #2522

[codecov]

Codecov Report

Merging #3425 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3425      +/-   ##
==========================================
- Coverage   72.73%   72.73%   -0.01%     
==========================================
  Files         807      807              
  Lines      145206   145206              
  Branches    16230    16230              
==========================================
- Hits       105618   105609       -9     
- Misses      35171    35179       +8     
- Partials     4417     4418       +1

Flag	Coverage Δ
#Debug	72.73% <ø> (-0.01%)	⬇️
#production	68.27% <ø> (-0.01%)	⬇️
#test	88.98% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/Microsoft.ML.FastTree/GamClassification.cs	89% <ø> (ø)	⬆️
src/Microsoft.ML.FastTree/TreeTrainersCatalog.cs	94.18% <ø> (ø)	⬆️
src/Microsoft.ML.Maml/MAML.cs	24.75% <0%> (-1.46%)	⬇️
src/Microsoft.ML.Transforms/Text/LdaTransform.cs	89.26% <0%> (-0.63%)	⬇️
...soft.ML.TestFramework/DataPipe/TestDataPipeBase.cs	73.73% <0%> (-0.33%)	⬇️
...ML.Data/Transforms/ColumnConcatenatingEstimator.cs	80.3% <0%> (ø)	⬆️
...soft.ML.Transforms/Text/WordEmbeddingsExtractor.cs	87.52% <0%> (ø)	⬆️
src/Microsoft.ML.Data/Transforms/Hashing.cs	92.32% <0%> (ø)	⬆️
...icrosoft.ML.FastTree/RandomForestClassification.cs	78.07% <0%> (ø)	⬆️
...forms/FeatureContributionCalculationTransformer.cs	73.55% <0%> (ø)	⬆️
... and 35 more

[shmoradims]