[Compatibility] Enforce CS1591 to document all public APIs

[carlossanlop]

CS1591 has severity set to suggestion in the root .editorconfig:

sdk/.editorconfig

Line 268 in 35a8299

dotnet_diagnostic.CS1591.severity = suggestion

We should change it to warning so that public APIs are triple-slash documented.

There might be other infra configuration changes to ensure docs work properly at the repo level.

[marcpopMSFT]

Is this for the api compat area in particular? The rest of the SDK doesn't release packages on nuget.org for consumption so doesn't have an api surface area intended to be used.

[carlossanlop]

Compatibility. Let me change the title.

[dotnet-policy-service]

@dotnet/area-infrastructure-libraries a new issue has been filed in the ApiCompat area, please triage