Currently (2/2017), there really are no good, free, and complete tools for data modeling and documenting your data model design, assuming you are using 10.3 or above. By far the best freebie is X-Ray, which allows round trip changes, batch changes, and minimal reporting. You edit the XML in Excel, with each Feature Class/Table as a tab. The bummer here is that reporting part is weak; you can't export to Visio and create a poster to mark up in a review process.
Esri Case Tools is good for this with Visio, but you have to make all changes in the UML in Visio (i.e. no round trips). Plus you probably have to use old versions of Visio, but I cannot say that with certainty.
ArcGIS Geodatabase Design with UML for Enterprise Architect is the most complete tool (graphics, round-trip), but you have to pay for EA, and honestly X-ray is quicker to get from A to B.
ArcGIS Diagrammer is kaput after 10.2 and never was better than the other options in my opinion, and Geodatabase Diagrammer is awesome, but getting it to work is nightmare. Little documentation to work through all the bugs/install process and confusing blog entries and multiple versions makes what should be a simple process turn into a wasted day. You may need to set up a secondary computer just to get the right versions running. There are not enough releases out there to account for all the different versions of Visio and ArcGIS dlls, meaning if you have Visio 2007 and ArcGIS 10.1, you are good. If you have 10.3.1 and Visio 2016 (like me), there is no Add-In compatible (that I have found); please correct me if I am wrong.
In short, X-Ray is the way to go even though you will have to suffer on the reporting capabilities by using the weak Graphics report options that come with it, until something better comes out.