add detail for gpu_id, document_fix (#32444)

python/paddle/fluid/framework.py

@@ -6073,7 +6073,8 @@ def device_guard(device=None):
     A context manager that specifies the device on which the OP will be placed.
 
     Args:
-        device(str|None): Specify the device to use in the context. It should be 'cpu' or 'gpu',
+        device(str|None): Specify the device to use in the context. It should be ``cpu``,
+            ``gpu`` or ``gpu:x``, where ``x`` is the index of the GPUs. 
             When it is set to 'cpu' or 'gpu', all OPs created in the context will be
             placed on CPUPlace or CUDAPlace. When 'gpu' is set and the program runs on
             single-card, the device index will be the same as the device on which the