[3.8] bpo-38600: NULL -> ``NULL``. (GH-17001) (GH-17003)

Also fix some other formatting.
(cherry picked from commit e835b31d2b212c3c7820364398979cae2a9740b2)
diff --git a/Doc/c-api/object.rst b/Doc/c-api/object.rst
index 992dca6..404a98f 100644
--- a/Doc/c-api/object.rst
+++ b/Doc/c-api/object.rst
@@ -354,7 +354,7 @@
    To get actual number of arguments, use
    :c:func:`PyVectorcall_NARGS(nargsf) <PyVectorcall_NARGS>`.
 
-   *kwnames* can be either NULL (no keyword arguments) or a tuple of keyword
+   *kwnames* can be either ``NULL`` (no keyword arguments) or a tuple of keyword
    names. In the latter case, the values of the keyword arguments are stored
    in *args* after the positional arguments.
    The number of keyword arguments does not influence *nargsf*.