Documenting doctest_default_flags config variable (#2471).

doc/ext/doctest.rst

@@ -55,13 +55,10 @@ names.
 
 .. rst:directive:: .. doctest:: [group]
 
-   A doctest-style code block.  You can use standard :mod:`doctest` flags for
-   controlling how actual output is compared with what you give as output.  By
-   default, these options are enabled: ``ELLIPSIS`` (allowing you to put
-   ellipses in the expected output that match anything in the actual output),
-   ``IGNORE_EXCEPTION_DETAIL`` (not comparing tracebacks),
-   ``DONT_ACCEPT_TRUE_FOR_1`` (by default, doctest accepts "True" in the output
-   where "1" is given -- this is a relic of pre-Python 2.2 times).
+   A doctest-style code block.  You can use standard :mod:`doctest`
+   flags for controlling how actual output is compared with what you
+   give as output.  The default set of flags is specified by the
+   :confval:`doctest_default_flags` configuration variable.
 
    This directive supports two options:
 
@@ -176,6 +173,16 @@ The following is an example for the usage of the directives.  The test via
 
 There are also these config values for customizing the doctest extension:
 
+.. confval:: doctest_default_flags
+
+   By default, these options are enabled: ``ELLIPSIS`` (allowing you
+   to put ellipses in the expected output that match anything in the
+   actual output), ``IGNORE_EXCEPTION_DETAIL`` (everything following
+   the leftmost colon and any module information in the exception name
+   is ignored), ``DONT_ACCEPT_TRUE_FOR_1`` (by default, doctest
+   accepts "True" in the output where "1" is given -- this is a relic
+   of pre-Python 2.2 times).
+
 .. confval:: doctest_path
 
    A list of directories that will be added to :data:`sys.path` when the doctest