updated docs

docs/LightningModule/RequiredTrainerInterface.md

@@ -136,7 +136,17 @@ def training_step(self, data_batch, batch_nb):
     
     # return a dict
     return output
-```
+```    
+
+If you define multiple optimizers, this step will also be called with an additional ```optimizer_idx``` param.    
+``` {.python}
+# Multiple optimizers (ie: GANs)     
+def training_step(self, data_batch, batch_nb, optimizer_idx):
+    if optimizer_idx == 0:
+        # do training_step with encoder
+    if optimizer_idx == 1:
+        # do training_step with decoder    
+```    
 
 --- 
 ### tng_dataloader