XML documentation

__Proposal:__
Add [XML documentation](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/xmldoc/) to all types, methods, etc.

__Current behavior:__
No XML documentation in the libraries.

__Desired behavior:__
All APIs, classes, etc. should be annotated with XML documentation comments.

The issue does not name files, tests, or entry points; start by identifying the library projects and their public types and methods. Review the linked XML documentation guidance and inventory the undocumented APIs. Done means all public APIs, classes, and related members have XML documentation comments.

Written by the indexing model from the issue text.