gh-151416: Document the __fspath__ reentrancy hazard of the FS path C APIs

PyUnicode_FSConverter(), PyUnicode_FSDecoder(), and PyOS_FSPath() call
an object's __fspath__() method during conversion, so the caller must
protect the input object with a strong reference that this method
cannot release.  The docs previously described only the result
reference semantics and gave no hint that passing a reference borrowed
from a mutable container is unsafe.

Author: gpshead

Doc/c-api/sys.rst

@@ -16,6 +16,11 @@ Operating System Utilities
    :class:`str` or :class:`bytes` object. Otherwise :exc:`TypeError` is raised
    and ``NULL`` is returned.
 
+   .. note::
+
+      The caller must hold a :term:`strong reference` to *path* that
+      its :meth:`~os.PathLike.__fspath__` method cannot release.
+
    .. versionadded:: 3.6
 
 

Doc/c-api/unicode.rst

@@ -925,6 +925,13 @@ conversion function:
    If *obj* is ``NULL``, the function releases a strong reference
    stored in the variable referred by *result* and returns ``1``.
 
+   .. note::
+
+      If *obj* is not a :class:`str` or :class:`bytes` instance, the
+      conversion calls its :meth:`~os.PathLike.__fspath__` method.  The
+      caller must hold a :term:`strong reference` to *obj* that this
+      method cannot release.
+
    .. versionadded:: 3.1
 
    .. versionchanged:: 3.6
@@ -952,6 +959,13 @@ conversion function:
    If *obj* is ``NULL``, release the strong reference
    to the object referred to by *result* and return ``1``.
 
+   .. note::
+
+      If *obj* is not a :class:`str` or :class:`bytes` instance, the
+      conversion calls its :meth:`~os.PathLike.__fspath__` method.  The
+      caller must hold a :term:`strong reference` to *obj* that this
+      method cannot release.
+
    .. versionadded:: 3.2
 
    .. versionchanged:: 3.6