[3.15] Mention PyUnicode_EncodeFSDefault in PyConfig.filesystem_encoding docs (GH-159297) (GH-159541)

Mention `PyUnicode_EncodeFSDefault` in `PyConfig.filesystem_encoding` docs (GH-159297)
(cherry picked from commit 49fc7ed)

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Author: miss-islington

Doc/c-api/init_config.rst

@@ -1430,6 +1430,10 @@ PyConfig
 
       See also the :c:member:`~PyConfig.filesystem_errors` member.
 
+      See also :c:func:`PyUnicode_EncodeFSDefault` and
+      :ref:`related functions <capi-filesystem-encoding>`
+      for encoding paths at runtime.
+
    .. c:member:: wchar_t* filesystem_errors
 
       :term:`Filesystem error handler <filesystem encoding and error handler>`:

Doc/c-api/unicode.rst

@@ -928,6 +928,8 @@ system.
       used for ``strict``.
 
 
+.. _capi-filesystem-encoding:
+
 File System Encoding
 """"""""""""""""""""