bpo-28677: Improve phrasing of when instance attribute is referenced (GH-6208)

Author: aaronang

Doc/tutorial/classes.rst

@@ -387,8 +387,8 @@ the corresponding function with an argument list that is created by inserting
 the method's instance object before the first argument.
 
 If you still don't understand how methods work, a look at the implementation can
-perhaps clarify matters.  When an instance attribute is referenced that isn't a
-data attribute, its class is searched.  If the name denotes a valid class
+perhaps clarify matters.  When a non-data attribute of an instance is
+referenced, the instance's class is searched.  If the name denotes a valid class
 attribute that is a function object, a method object is created by packing
 (pointers to) the instance object and the function object just found together in
 an abstract object: this is the method object.  When the method object is called