add desc for minimize (#3216)

docs/source/results.rst

@@ -80,6 +80,10 @@ The `TrainResult` basic usage is this:
 
 minimize
 ^^^^^^^^
+When using TrainResult, the metric that needs to be minimized is passed to this
+argument. Internally, the tensor is verified to contain gradients and `.backward()`
+is called on it.
+
 .. code-block:: python
 
     def training_step(...):