Document default values of TrainerOptions #6450

Is your feature request related to a problem? Please describe.
Looking up the default value of LightGbmMulticlassTrainer.Options.NumberOfIterations just took me way too long. The default values should be stated in the documentation.

Describe the solution you'd like
The documentation should mention the default values.

Describe alternatives you've considered
Create an application, instantiate the object there and print out the values.

Additional context
I reported this before in #4311. There, instead of updating the documentation, I was provided the answer. That's nice, but not scalable, and wasn't my suggestion. To be clear: I suggest you include all default values in the documentation.