Document default values of TrainerOptions

**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.