It is always good if you mention the probation period, termination clause during probation, notice period once confirmed, location of initial posting, transfer of employment, period of validity of the offer (by which date the employee should join or by which date the employee should accept the offer) and clause of medical fitness with a condition regarding termination of offer in case the candidate fails in the medical examination.
An offer once accepted and signed by the employee becomes a contract and therefore, there is no need to give another letter elaborating the salary and other terms and conditions of employment. If you include everything in the offer letter itself, it will become a transparent offer and the employee can accept it without any reservations. At the same time, if you only mention about, say gross salary or the Cost to Company, the candidate may not be able to get any idea about the take home salary that he is going to get by the end of the month. If the CTC includes performance based incentive, there may arise disputes with regard to the actual remuneration offered. If you mention the salary bifurcation with take home part, the variable part, contribution by the employer towards PF etc separately in the offer letter itself, it will become a conclusive proof of consensus ad idem.