Document rb_get_kwargs

[FnControlOption]

rb_get_kwargs is useful with rb_scan_args

[FnControlOption]

https://github.com/ruby/ruby/blob/cb4e2cb55a59833fc4f1f6db2f3082d1ffcafc80/include/ruby/internal/eval.h#L340-L342

 * @warning     Handling keyword arguments in the  C API is less efficient than
 *              handling them  in Ruby.  Consider  using a Ruby  wrapper method
 *              around a non-keyword C function.