document that sys._is_gil_enabled is CPython-specific (pythonGH-131748)

(cherry picked from commit ac12de2)

Co-authored-by: Ralf Gommers <ralf.gommers@gmail.com>

Author: rgommers

Doc/library/sys.rst

@@ -1235,6 +1235,9 @@ always available. Unless explicitly noted otherwise, all variables are read-only
 
    .. versionadded:: 3.13
 
+   .. impl-detail::
+
+      It is not guaranteed to exist in all implementations of Python.
 
 .. function:: is_finalizing()