Improve the internal documentation of operation validation

In operation_validation.mli,

• Explain the motivation behind the the validation module in general.
• Explain also the minimal requirements/spec for adding new manager operations
• Document the different preconditions and checks in the validation (See #2603 (closed))
• Document the intended usage of precheck_manager and prechecker_manager_no_validation_state and explain that they are legacy and intended to be deleted.