Add PyUnicode_AppendAndDel function doc

Author: rruuaanng

Doc/c-api/unicode.rst

@@ -615,6 +615,12 @@ APIs:
    On error, set *p_left* to ``NULL`` and set an exception.
 
 
+.. c:function:: void PyUnicode_AppendAndDel(PyObject **p_left, PyObject *right)
+
+   The function is identical to :c:func:`PyUnicode_Append`, with the only
+   difference being that it decrements the reference count of *right* by one.
+
+
 .. c:function:: const char* PyUnicode_GetDefaultEncoding(void)
 
    Return the name of the default string encoding, ``"utf-8"``.