[3.13] gh-46236: Add docs for PyUnicode_GetDefaultEncoding() doc (GH-130335) (GH-130511)

* Clarify sys.getdefaultencoding() documentation

* Add missing documentation for PyUnicode_GetDefaultEncoding,
  the C equivalent of sys.getdefaultencoding
(cherry picked from commit 9f25c1f012)

Co-authored-by: RUANG (James Roy) <longjinyii@outlook.com>

Doc/c-api/unicode.rst

@@ -607,6 +607,15 @@ APIs:
    decref'ing the returned objects.
 
 
+.. c:function:: const char* PyUnicode_GetDefaultEncoding(void)
+
+   Return the name of the default string encoding, ``"utf-8"``.
+   See :func:`sys.getdefaultencoding`.
+
+   The returned string does not need to be freed, and is valid
+   until interpreter shutdown.
+
+
 .. c:function:: Py_ssize_t PyUnicode_GetLength(PyObject *unicode)
 
    Return the length of the Unicode object, in code points.

Doc/data/refcounts.dat

@@ -2756,6 +2756,9 @@ PyUnicode_FromFormatV:PyObject*::+1:
 PyUnicode_FromFormatV:const char*:format::
 PyUnicode_FromFormatV:va_list:args::
 
+PyUnicode_GetDefaultEncoding:const char*:::
+PyUnicode_GetDefaultEncoding::void::
+
 PyUnicode_GetLength:Py_ssize_t:::
 PyUnicode_GetLength:PyObject*:unicode:0:
 

Doc/library/sys.rst

@@ -764,8 +764,8 @@ always available. Unless explicitly noted otherwise, all variables are read-only
 
 .. function:: getdefaultencoding()
 
-   Return the name of the current default string encoding used by the Unicode
-   implementation.
+   Return ``'utf-8'``. This is the name of the default string encoding, used
+   in methods like :meth:`str.encode`.
 
 
 .. function:: getdlopenflags()