Looking at your second question, You can add instructions to your request types by completing the 'detailed Instructions' field on the request type setup. This then appears as a green box when you select the request type.
If you're looking to add instructions for specific custom fields, you can complete the Info field on the custom field setup.
Your first question; to customise the client history tab would require embedding custom code which is not supported if you run into any issues (though developers may try to assist in diagnosing), it isn't recommended, and it's more fiddly than it's worth (it requires adding and updating every time you migrate, upgrade or alter your help desk in any way shape or form). This all being said, it is possible but through personal experience, isn't worth it unless you're making Web Help Desk a legacy software in your company.