DiagnosticSource Needs Documentation

[brianrob]

As @NickCraver points out in #20992, we need documentation that describes how DiagnosticSource is supposed to work from event production to consumption. Further, this documentation should describe from a user's perspective what is required and expected.

[danmoseley]

If this is for docs.dot.net the issue should be in https://github.com/dotnet/docs I think

[brianrob]

It's not yet clear where this documentation should live, but should we determine it to be docs.dot.net, I can move it.

[wtgodbe]

@brianrob can you move this issue to the docs repo? None of our framework documentation lives in this repo

[mikelle-rogers]

DiagnosticListener needs documentation as well. See dotnet diagnostics 515