Altium Document/Schematic Option Revision field auto value from Subversion
Use the special string =VersionControl_RevNumber. 

You can find a full list of strings on Altium documentation. The strings are separate for Shecmatics and PCB.

Schematic special strings start with a = and PCB special strings start with a .. When you start typing, Altium will give you suggestions for the available strings, so you can find the right one even if you don't know the full string.