Microsoft has a world of "XRM" within Dynamics CRM and as more and more of the framework matures we will see growth.
On my latest deep dive I have been looking at modifying Dynamics CRM Help and the general structure around how changes to Dynamics CRM help are supported in a multi-tenant world. In summary they are not (yet). The help files are stored with the application files.
But luckily there is some great reading material to get started with changes you might want to make to help in a single tenant world and customizing help is supported. You can read the complete Microsoft Dynamics CRM Configuration Guide on Technet and when talking HELP you can focus on this section.