Fix up docs for MLContext

[gewarren]

Cleans up the public-facing API docs for MLContext to conform to https://github.com/dotnet/dotnet-api-docs/wiki.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.89%. Comparing base (01c4164) to head (6c6caef).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7334      +/-   ##
==========================================
- Coverage   68.90%   68.89%   -0.01%     
==========================================
  Files        1473     1473              
  Lines      274277   274277              
  Branches    28421    28421              
==========================================
- Hits       188983   188972      -11     
- Misses      77975    77985      +10     
- Partials     7319     7320       +1     

Flag	Coverage Δ
Debug	68.89% <ø> (-0.01%)	⬇️
production	63.30% <ø> (-0.01%)	⬇️
test	89.43% <ø> (ø)

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Files with missing lines	Coverage Δ
src/Microsoft.ML.Data/MLContext.cs	85.71% <ø> (ø)

... and 5 files with indirect coverage changes

[ericstj]

Looks reasonable, thank you @gewarren

[ericstj]

/ba-g test failure is #7339

[michaelgsharp]

/backport to release/4.0

[github-actions]

Started backporting to release/4.0: https://github.com/dotnet/machinelearning/actions/runs/12777796720