Fixed an indentation issue and updated a paragraph on the usage of isinstance().

docs/src/userguide/extension_types.rst

@@ -129,7 +129,7 @@ which requires the use of a local variable and performs a type test on assignmen
 If you *know* the return value of :meth:`quest` will be of type :class:`Shrubbery`
 you can use a cast to write::
 
-    print((<Shrubbery>quest()).width)
+    print( (<Shrubbery>quest()).width )
 
 This may be dangerous if :meth:`quest()` is not actually a :class:`Shrubbery`, as it
 will try to access width as a C struct member which may not exist. At the C level,
@@ -137,21 +137,17 @@ rather than raising an :class:`AttributeError`, either an nonsensical result wil
 returned (interpreting whatever data is at that address as an int) or a segfault
 may result from trying to access invalid memory. Instead, one can write::
 
-    print((<Shrubbery?>quest()).width)
+    print( (<Shrubbery?>quest()).width )
 
 which performs a type check (possibly raising a :class:`TypeError`) before making the
 cast and allowing the code to proceed.
 
-To explicitly test the type of an object, use the :meth:`isinstance` method. By default,
-in Python, the :meth:`isinstance` method checks the :class:`__class__` attribute of the
-first argument to determine if it is of the required type. However, this is potentially
-unsafe as the :class:`__class__` attribute can be spoofed or changed, but the C structure
-of an extension type must be correct to access its :keyword:`cdef` attributes and call its :keyword:`cdef` methods. Cython detects if the second argument is a known extension
-type and does a type check instead, analogous to Pyrex's :meth:`typecheck`.
-The old behavior is always available by passing a tuple as the second parameter::
-
-    print(isinstance(sh, Shrubbery))     # Check the type of sh
-    print(isinstance(sh, (Shrubbery,)))  # Check sh.__class__
+To explicitly test the type of an object, use the :meth:`isinstance` method.
+For known builtin or extension types, Cython translates these into a
+fast and safe type check that ignores changes to
+the object's ``__class__`` attribute etc., so that after a successful
+:meth:`isinstance` test, code can rely on the expected C structure of the
+extension type and its :keyword:`cdef` attributes and methods.
 
 
 Extension types and None