#3229 fix typo and expand notes a little

Doc/reference/compound_stmts.rst

@@ -569,12 +569,13 @@ is equivalent to ::
    Foo = f1(arg)(f2(Foo))
 
 **Programmer's note:** Variables defined in the class definition are class
-can be set in a method with ``self.name = value``.  Both class and instance
-variables are accessible through the notation "``self.name``", and an instance
-variable hides a class variable with the same name when accessed in this way.
-Class variables can be used as defaults for instance variables, but using
-mutable values there can lead to unexpected results.  Descriptors can be used
-to create instance variables with different implementation details.
+variables; they are shared by instances. Instance variables can be set in a
+method with ``self.name = value``.  Both class and instance variables are
+accessible through the notation "``self.name``", and an instance variable hides
+a class variable with the same name when accessed in this way.  Class variables
+can be used as defaults for instance variables, but using mutable values there
+can lead to unexpected results.  Descriptors can be used to create instance
+variables with different implementation details.
 
 .. XXX add link to descriptor docs above