Add explanation of the parameters to _infer_type_vars

jedi/inference/gradual/annotation.py

@@ -331,6 +331,27 @@ def _infer_type_vars(annotation_value, value_set, is_class_value=False):
 
     This functions would generate `int` for `_T` in this case, because it
     unpacks the `Iterable`.
+
+    Parameters
+    ----------
+
+    `annotation_value`: represents the annotation of the current parameter to
+        infer the value for. In the above example, this would initially be the
+        `Iterable[_T]` of the `iterable` parameter and then, when recursing,
+        just the `_T` generic parameter.
+
+    `value_set`: represents the actual argument passed to the parameter we're
+        inferrined for, or (for recursive calls) their types. In the above
+        example this would first be the representation of the list `[1]` and
+        then, when recursing, just of `1`.
+
+    `is_class_value`: tells us whether or not to treat the `value_set` as
+        representing the instances or types being passed, which is neccesary to
+        correctly cope with `Type[T]` annotations. When it is True, this means
+        that we are being called with a nested portion of an annotation and that
+        the `value_set` represents the types of the arguments, rather than their
+        actual instances.
+        Note: not all recursive calls will neccesarily set this to True.
     """
     type_var_dict = {}
     annotation_name = annotation_value.py__name__()