Add missing XML documentation to public APIs #7788

Goal

Add missing XML documentation to public APIs, including publicly accessible protected members, so that IntelliSense and generated API reference documentation cover the API surface.

Basic inventory

A diagnostic build at commit 272f2fb found 6,459 distinct types/members missing XML documentation in 34 projects, plus 238 additional undocumented members suppressed by source-level pragmas.

Stable-version projects (1.x and later)

Pre-1.0 projects (0.x)

Main concentrations and examples

• Data.Analysis: 4,898 of its 5,153 gaps are in two T4-generated files: PrimitiveDataFrameColumn.BinaryOperationAPIs.ExplodedColumns.cs (3,151) and PrimitiveDataFrameColumn.BinaryOperators.cs (1,747). Update the corresponding .tt templates and regenerate their outputs rather than hand-editing generated files.
• Source-level suppression: another 238 distinct Data.Analysis members are missing documentation in DataFrame.BinaryOperators.cs and DataFrameColumn.BinaryOperators.cs, where #pragma warning disable 1591 remains active. These are not included in the tables.
• AutoML: 227 of its 371 gaps are in source-generator output; these should be addressed in the generator.
• Ordinary API gaps: examples include AutoMLExperiment's constructor and SetTrainingTimeInSeconds, DataViewRowId.CompareTo and Equals(DataViewRowId), and the span-based BertTokenizer.CreateTokenTypeIdsFromSequences overload. The last example is the only gap found in Tokenizers.

Incremental enforcement

CS1591 could be enabled in an individual project once all of its APIs are documented, without waiting for the entire repository to be clean.

For each completed project, remove its effective CS1591 suppression, including the suppression inherited from src/Directory.Build.props and any project-specific or source-level suppressions. Once clean, also remove CS1591 from its effective WarningsNotAsErrors so that the existing warnings-as-errors policy prevents new documentation gaps. This should be done selectively for completed projects while unfinished projects retain their suppressions.