Clarify what gpus=0 means in docs (#2876)

* docs clarify what gpus=0 means

* add example suggested by @ydcjeff

pytorch_lightning/trainer/__init__.py

@@ -414,8 +414,8 @@ Under the hood the pseudocode looks like this:
 gpus
 ^^^^
 
-- Number of GPUs to train on
-- or Which GPUs to train on
+- Number of GPUs to train on (int)
+- or which GPUs to train on (list)
 - can handle strings
 
 .. testcode::
@@ -423,6 +423,9 @@ gpus
     # default used by the Trainer (ie: train on CPU)
     trainer = Trainer(gpus=None)
 
+    # equivalent
+    trainer = Trainer(gpus=0)
+
 Example::
 
     # int: train on 2 gpus
@@ -440,6 +443,9 @@ Example::
     # uses 8 gpus in total
     trainer = Trainer(gpus=2, num_nodes=4)
 
+    # train only on GPUs 1 and 4 across nodes
+    trainer = Trainer(gpus=[1, 4], num_nodes=4)
+
 See Also:
     - `Multi-GPU training guide <multi_gpu.rst>`_