How much information should be disclosed in a letter of resignation about...?

However, I am being asked all sorts of questions by senior management and the HR about my new job like:

1. Which company are you joining. What do they do.
2. How much are they paying you (details).
3. Why do you think the work there will be better than here.
4. What other benefits are you receiving. Are they providing you facility X .

None of this is compulsory for me to answer, but I don't like pretending to be vague or reluctant all the time.

How truthfully should I answer the above questions. Not that there is any harm in being entirely truthful but I just don't feel too good about telling them every single detail.

Any suggestions. Why do companies ask for all these details about an employee's new job.