Mention PyUnicode_EncodeFSDefault in PyConfig.filesystem_encoding docs

Author: encukou

Doc/c-api/init_config.rst

@@ -1439,6 +1439,9 @@ PyConfig
 
       See also the :c:member:`~PyConfig.filesystem_errors` member.
 
+      See also :c:func:`PyUnicode_EncodeFSDefault` for encoding paths
+      at runtime.
+
    .. c:member:: wchar_t* filesystem_errors
 
       :term:`Filesystem error handler <filesystem encoding and error handler>`: