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/abstract.rst b/Doc/c-api/abstract.rst
index 0edd1d5..1823f9d 100644
--- a/Doc/c-api/abstract.rst
+++ b/Doc/c-api/abstract.rst
@@ -18,6 +18,7 @@
 .. toctree::
 
    object.rst
+   call.rst
    number.rst
    sequence.rst
    mapping.rst