Document new directive.
diff --git a/Doc/documenting/markup.rst b/Doc/documenting/markup.rst
index 4c56cd2..eec8650 100644
--- a/Doc/documenting/markup.rst
+++ b/Doc/documenting/markup.rst
@@ -626,6 +626,24 @@
--------------
+.. describe:: impl-detail
+
+ This directive is used to mark CPython-specific information. Use either with
+ a block content or a single sentence as an argument, i.e. either ::
+
+ .. impl-detail::
+
+ This describes some implementation detail.
+
+ More explanation.
+
+ or ::
+
+ .. impl-detail:: This shortly mentions an implementation detail.
+
+ "\ **CPython implementation detail:**\ " is automatically prepended to the
+ content.
+
.. describe:: seealso
Many sections include a list of references to module documentation or