Document changes in PyNode_AddChild and PyParser_AddToken (GH-14214)

I didn't find any entries in the docs about these functions, so I just mentioned them, in "What's New".

Doc/whatsnew/3.8.rst

@@ -1457,6 +1457,8 @@ Changes in the C API
 * The :c:func:`PyCode_New` has a new parameter in the second position (*posonlyargcount*)
   to support :pep:`570`, indicating the number of positional-only arguments.
 
+* The functions :c:func:`PyNode_AddChild` and :c:func:`PyParser_AddToken` now accept
+  two additional ``int`` arguments *end_lineno* and *end_col_offset*.
 
 CPython bytecode changes
 ------------------------