Use "impl-detail" directive where applicable.
diff --git a/Doc/library/platform.rst b/Doc/library/platform.rst
index 98b7972..b98d992 100644
--- a/Doc/library/platform.rst
+++ b/Doc/library/platform.rst
@@ -98,7 +98,7 @@
 .. function:: python_implementation()
 
    Returns a string identifying the Python implementation. Possible return values
-   are: 'CPython', 'IronPython', 'Jython'
+   are: 'CPython', 'IronPython', 'Jython'.
 
    .. versionadded:: 2.6