Understanding the makeup of an XML .ev3p file
I personally doubt that there is any publicly available documentation of the contents of these files. However, you may gain some insight from the documentation in the "EV3 Block Developer Kit" available on the LEGO MINDSTORMS Download page.

Tip: rather than using the outdated copy of the EV3 software included in the EV3 Block Developer Kit, I have found that you can create a file named MindstormsEV3.ini with the following contents:

&lt;?xml version="1.0" encoding="utf-8" ?&gt; 
&lt;Tokens DeveloperMode="True" /&gt;


and save it in the LEGO MINDSTORMS EV3 program directory to enable developer mode on any installed version of the software.