Add a summary for each ExecutionProviderAppend methods in SessionOptions.cs (#3111) (#3112)
* Add a summary for each ExecutionProviderAppend methods in SessionOptions.cs
OnnxRuntime managed dll is EP agnostic meaning it will expose all methods pertaining to all possible EPs supported by OnnxRuntime in general. Not all these methods are really "available" to use for a .NET developer unless they have the correpsonding native onnxruntime shared library. Adding a summary line so that intellisense points that out.
* remove empty line