Add XML documentation to all public APIs

[jzills]

Summary

• Added and improved XML documentation comments (<summary>, <param>, <returns>, <exception>, <typeparam>) across all public types and members in the src/ directory
• Fixed inaccurate <summary> text (wrong cref class names, copy-pasted descriptions from unrelated types, blank summaries)
• Replaced redundant <remarks> that repeated the summary with <exception> tags where applicable (InvalidOperationException on lock timeout paths)
• Clarified units and semantics on expiration fields (AbsoluteExpiration as Unix ms, TTL as Cosmos DB seconds, etc.)

Test plan

• dotnet build src/ produces 0 warnings and 0 errors
• IntelliSense tooltips in IDE show accurate, complete descriptions for all changed types