Allow using GenerateDocumentationFile to produce docs from triple slash instead of using the commited xml file

[carlossanlop]

With this change, we can add new assemblies that can opt-in to using triple slash comments as source of truth, instead of a checked-in xml file, by simply setting GenerateDocumentationFile to true in the csproj.

I tested building and packaging using the temporary commit (will revert it before merging).

I confirmed that the xml for System.Buffers is generated and it is used instead of the commited file. I could find the generated xml in these 3 locations:

• artifacts/bin/System.Buffers/<Config>/<TargetFramework>/System.Buffers.xml
• artifacts/bin/System.Memory/<Config>/<TargetFramework>/System.Buffers.xml - This was allowed by bringing AnnotateTargetPathWithTargetPlatformMonikerWithDocFileItem.
• artifacts/packages/System.Buffers.4.6.0.nupkg/lib/<TargetFramework>/System.Buffers.xml

[ericstj]

LGTM: In addition to answering Viktor's question - make sure you add to your documentation PR the two options for documentation.

[carlossanlop]

LGTM: In addition to answering Viktor's question - make sure you add to your documentation PR the two options for documentation.

Thanks! Question answered, and I added a TODO reminder in the documentation PR.

[carlossanlop]

The test commit has been reverted, it was just to show how I tested it.