[Documentation] EventSource documentation improvement

[sywhang]

• current preferred patterns
• Increased detail on use cases of and differences between the various ctors on EventSource
• Discussion on the differences between the various types of EventSources, e.g., Self Describing vs TraceLogging, etc.
• Increased documentation on features such as custom keywords
• Clarification on differences between consumption channels, i.e., nuget package, BCL, redist nuget package
• EventSource.GetGuid can differ from EventSource.Guid (EventSource.GetGuid can differ from EventSource.Guid runtime#1360)

[mikelle-rogers]

Closing this issue because the last two tasks do not need to be completed.