C API docs: Py_IsInitialized is always safe to call (GH-12630)

(cherry picked from commit ddbb978e1065dde21d1662386b26ded359f4b16e)

Co-authored-by: Nick Coghlan <ncoghlan@gmail.com>
diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 50998be..2cdc052 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -37,6 +37,7 @@
 
 * Informative functions:
 
+  * :c:func:`Py_IsInitialized`
   * :c:func:`PyMem_GetAllocator`
   * :c:func:`PyObject_GetArenaAllocator`
   * :c:func:`Py_GetBuildInfo`