bpo-25381: Update explanation of exceptions in C. (GH-26838) (GH-29569)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu> Co-authored-by: Serhiy Storchaka <storchaka@gmail.com> (cherry picked from commit ad43dc0)

Author: miss-islington

Doc/extending/extending.rst

@@ -127,13 +127,11 @@ Intermezzo: Errors and Exceptions
 
 An important convention throughout the Python interpreter is the following: when
 a function fails, it should set an exception condition and return an error value
-(usually a ``NULL`` pointer). Exceptions are stored in a static global variable
-inside the interpreter; if this variable is ``NULL`` no exception has occurred. A
-second global variable stores the "associated value" of the exception (the
-second argument to :keyword:`raise`). A third variable contains the stack
-traceback in case the error originated in Python code. These three variables
-are the C equivalents of the result in Python of :meth:`sys.exc_info` (see the
-section on module :mod:`sys` in the Python Library Reference). It is important
+(usually ``-1`` or a ``NULL`` pointer). Exception information is stored in
+three members of the interpreter's thread state. These are ``NULL`` if
+there is no exception. Otherwise they are the C equivalents of the members
+of the Python tuple returned by :meth:`sys.exc_info`. These are the
+exception type, exception instance, and a traceback object. It is important
 to know about them to understand how errors are passed around.
 
 The Python API defines a number of functions to set various types of exceptions.

Misc/NEWS.d/next/Documentation/2021-06-21-17-51-51.bpo-25381.7Kn-_H.rst

@@ -0,0 +1,2 @@
+In the extending chapter of the extending doc, update a paragraph about the
+global variables containing exception information.