Verify doc comments specification

[daveaglick]

Make sure we're covering everything in https://github.com/dotnet/csharplang/blob/master/spec/documentation-comments.md

[savornicesei]

Hi @daveaglick

I've just extended XML comments support in Wyam through this PR
Since the CodeAnalysis project has not diverged too much from Wyam, I could create a PR for Statiq as well, if you're fine with the changes.
I'm also attaching the analysis doc I did related to XML comments support in std, docFX, SHFB and Wyam2.
Wyam2 - XML comments support.pdf

Best,
Simo