Not Sure that I understand. Kentico keeps track ob objects changes such as templates, forms, etc and stores those in the CMS_ObjectVersionHistory. There is a history of template modification for your document.
select Convert(XML,VersionXML),* from CMS_ObjectVersionHistory where VersionObjectType = 'cms.pagetemplate' and VersionObjectDisplayName ='YourDocumentTemplateName'
DocumentContent is the field in CMS_Document, that usually contains whatever is document related for ex.: 'Editable text' web part definition. So if you modify your text in 'Editable text' web part on your page - it will be stored in CMS_Document.DocumentContent, but I not sure that there is a history for such modifications.