Recognize backquotes and triple-backquotes in xml docs

[jcouv]

Relates to https://github.com/dotnet/roslyn/pull/23984/files#r178449741

[CyrusNajmabadi]

This would be nice to have :) Look at the DocumentationComment type for where the IDE takes the xml text and processes it.

[sharwell]

This needs to go to a design meeting. I have an objection to merging/shipping an implementation prior to design approval and adoption in the language specification.

[sharwell]

See dotnet/csharplang#891 for the language design issue.

[svick]

I agree that supporting Markdown (and possibly also some simple syntax for XML elements that don't have counterparts in Markdown) is a great idea. But it should be done all at once in a systematic manner, not "add backquotes, then some other syntax, then another, and end up with complex, undocumented mess".

[sharwell]

During a design meeting today, we determined that this feature can't really move forward until at minimum one of the following occurs:

• The language specification is updated to define generalized documentation comment support for the proposed syntax
• The Visual Studio editor/platform team puts forward a proposal for unified handling of this syntax across a variety of programming languages supported in Visual Studio, as a visual aid to developers who wish to deviate from features specific to individual languages

The implementation of the second item does not guarantee that the feature will be implemented without support from the language specification; should that situation arise will again consider the available design proposals to decide if the issue should remain blocked.