add document for method ChangeModelThreshold

[frank-dong-ms-zz]

Fixes issue #4527 , add documentation for public method ChnageModelThreshold.

This method is introduced in #2969

[codecov]

Codecov Report

Merging #4563 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4563      +/-   ##
==========================================
+ Coverage   75.12%   75.12%   +<.01%     
==========================================
  Files         909      909              
  Lines      160240   160240              
  Branches    17252    17252              
==========================================
+ Hits       120377   120378       +1     
+ Misses      35049    35048       -1     
  Partials     4814     4814

Flag	Coverage Δ
#Debug	75.12% <ø> (ø)	⬆️
#production	70.52% <ø> (ø)	⬆️
#test	90.28% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
src/Microsoft.ML.Data/TrainCatalog.cs	82.11% <ø> (ø)	⬆️
src/Microsoft.ML.AutoML/Sweepers/ISweeper.cs	59.49% <0%> (-10.13%)	⬇️
....ML.AutoML/PipelineSuggesters/PipelineSuggester.cs	83.19% <0%> (-3.37%)	⬇️
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	84.32% <0%> (-0.85%)	⬇️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	85.11% <0%> (-0.21%)	⬇️
src/Microsoft.ML.Maml/MAML.cs	26.21% <0%> (+1.45%)	⬆️
...L.AutoML/TrainerExtensions/TrainerExtensionUtil.cs	86.89% <0%> (+1.74%)	⬆️
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	65.56% <0%> (+5.96%)	⬆️