bpo-36974: expand call protocol documentation (GH-13844)



CC @encukou 

I'm also adding Petr Viktorin as contributor for vectorcall in the "what's new" section.


https://bugs.python.org/issue36974



Automerge-Triggered-By: @encukou

Automerge-Triggered-By: @encukou
diff --git a/Doc/c-api/structures.rst b/Doc/c-api/structures.rst
index 1035277..1bd769f 100644
--- a/Doc/c-api/structures.rst
+++ b/Doc/c-api/structures.rst
@@ -208,7 +208,8 @@
 
    Extension of :const:`METH_FASTCALL` supporting also keyword arguments,
    with methods of type :c:type:`_PyCFunctionFastWithKeywords`.
-   Keyword arguments are passed the same way as in the vectorcall protocol:
+   Keyword arguments are passed the same way as in the
+   :ref:`vectorcall protocol <vectorcall>`:
    there is an additional fourth :c:type:`PyObject\*` parameter
    which is a tuple representing the names of the keyword arguments
    (which are guaranteed to be strings)