Is there already existing a tool , I think "documentor" can be his name for Sharepoint 2010 that can output a form of documentation with a description of all db, web application , sites, sub sites , users, configurations , permissions etc. existing on a SharePoint Server ? Or should I go create one using PowerShell ?
Check out the Documentation Toolkit for SharePoint. It's a really good tool that documents your farm and its configuration and even changes made to configurations.