Not sure about documentation but can tell you from what I see in my lab 5596. Module 1 is the base fixed ports. Top left is module 2, the middle one is module 3 and the top right module 4. In my switch I have a L3 card in top left slot, Ethernet module in middle slot and right slot is empty.
prkrishn-Nexus5596-6# show mod
Mod Ports Module-Type Model Status
--- ----- -------------------------------- ---------------------- ------------
1 48 O2 48X10GE/Modular Supervisor N5K-C5596UP-SUP active *
2 32 O2 GEM with L3 ASIC N55-M160L3 ok
3 16 O2 16X10GE Ethernet Module N51-M16EP ok
Hope this helps.