Fix up docs

[gewarren]

I made a bunch of improvements to the docs and spelling.

• For properties, you should use <value> instead of <remarks> to provide the default value.
• For exceptions, word it as if the sentence starts with "This exception is thrown if".

More guidance is here: https://github.com/dotnet/dotnet-api-docs/wiki

Microsoft Reviewers: Open in CodeFlow