gh-90241: Clarify documentation for PyUnicode_FSConverter and PyUnicode_FSDecoder

Author: serhiy-storchaka

Doc/c-api/arg.rst

@@ -307,7 +307,7 @@ Other objects
 
 .. _o_ampersand:
 
-``O&`` (object) [*converter*, *anything*]
+``O&`` (object) [*converter*, *address*]
    Convert a Python object to a C variable through a *converter* function.  This
    takes two arguments: the first is a function, the second is the address of a C
    variable (of arbitrary type), converted to :c:expr:`void *`.  The *converter*
@@ -327,6 +327,9 @@ Other objects
    call, the *object* parameter will be ``NULL``; *address* will have the same value
    as in the original call.
 
+   Examples of converters: :c:func:`PyUnicode_FSConverter` and
+   :c:func:`PyUnicode_FSDecoder`.
+
    .. versionchanged:: 3.1
       ``Py_CLEANUP_SUPPORTED`` was added.
 

Doc/c-api/unicode.rst

@@ -786,33 +786,48 @@ Functions encoding to and decoding from the :term:`filesystem encoding and
 error handler` (:pep:`383` and :pep:`529`).
 
 To encode file names to :class:`bytes` during argument parsing, the ``"O&"``
-converter should be used, passing :c:func:`PyUnicode_FSConverter` as the
+converter should be used, passing :c:func:`!PyUnicode_FSConverter` as the
 conversion function:
 
 .. c:function:: int PyUnicode_FSConverter(PyObject* obj, void* result)
 
-   ParseTuple converter: encode :class:`str` objects -- obtained directly or
+   :ref:`PyArg_Parse\* converter <arg-parsing>`: encode :class:`str` objects -- obtained directly or
    through the :class:`os.PathLike` interface -- to :class:`bytes` using
    :c:func:`PyUnicode_EncodeFSDefault`; :class:`bytes` objects are output as-is.
-   *result* must be a :c:expr:`PyBytesObject*` which must be released when it is
-   no longer used.
+   *result* must be an address of a C variable of type :c:expr:`PyObject*`
+   (or :c:expr:`PyBytesObject*`).
+   The variable is set to a new :term:`strong reference` to
+   a :ref:`bytes object <bytesobjects>` which must be released
+   when it is no longer used.
+   Embedded null bytes are not allowed in the result.
+
+   If *obj* is ``NULL``, the function releases a strong reference
+   stored in the variable referred by *result*.
 
    .. versionadded:: 3.1
 
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.
 
 To decode file names to :class:`str` during argument parsing, the ``"O&"``
-converter should be used, passing :c:func:`PyUnicode_FSDecoder` as the
+converter should be used, passing :c:func:`!PyUnicode_FSDecoder` as the
 conversion function:
 
 .. c:function:: int PyUnicode_FSDecoder(PyObject* obj, void* result)
 
-   ParseTuple converter: decode :class:`bytes` objects -- obtained either
+   :ref:`PyArg_Parse\* converter <arg-parsing>`: decode :class:`bytes` objects -- obtained either
    directly or indirectly through the :class:`os.PathLike` interface -- to
    :class:`str` using :c:func:`PyUnicode_DecodeFSDefaultAndSize`; :class:`str`
-   objects are output as-is. *result* must be a :c:expr:`PyUnicodeObject*` which
-   must be released when it is no longer used.
+   objects are output as-is.
+   *result* must be an address of a C variable of type :c:expr:`PyObject*`
+   (or :c:expr:`PyUnicodeObject*`).
+   The variable is set to a new :term:`strong reference` to
+   a :ref:`Unicode object <unicodeobjects>` which must be released
+   when it is no longer used.
+   Embedded null characters are not allowed in the result.
+
+   If *obj* is ``NULL``, the function releases a strong reference
+   stored in the variable referred by *result*.
 
    .. versionadded:: 3.2