Update Graph docstring to match __init__.py (#48100)

Summary: Pull Request resolved: #48100

Test Plan: Imported from OSS

Reviewed By: jamesr66a

Differential Revision: D25023407

Pulled By: ansley

fbshipit-source-id: e00706059b4c684451d2e1e48ca634b42693c1e1

torch/fx/graph.py

@@ -153,7 +153,8 @@ def forward(self, x):
     The Node semantics are as follows:
 
     - `placeholder` represents a function input. The `name` attribute specifies the name this value will take on.
-    `target` is similarly the name of the argument. `args` and `kwargs` are don't-care. Placeholders correspond to
+    `target` is similarly the name of the argument. `args` holds either: 1) nothing, or 2) a single argument
+    denoting the default parameter of the function input. `kwargs` is don't-care. Placeholders correspond to
     the function parameters (e.g. `x`) in the graph printout.
     - `get_attr` retrieves a parameter from the module hierarchy. `name` is similarly the name the result of the
     fetch is assigned to. `target` is the fully-qualified name of the parameter's position in the module hierarchy.