bpo-38600: Change the mark up of NULL in the C API documentation. (GH-16950)

Replace all *NULL* with ``NULL``.
diff --git a/Doc/c-api/gcsupport.rst b/Doc/c-api/gcsupport.rst
index b3f30b2..924a7fd 100644
--- a/Doc/c-api/gcsupport.rst
+++ b/Doc/c-api/gcsupport.rst
@@ -49,7 +49,7 @@
 .. c:function:: TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize)
 
    Resize an object allocated by :c:func:`PyObject_NewVar`.  Returns the
-   resized object or *NULL* on failure.  *op* must not be tracked by the collector yet.
+   resized object or ``NULL`` on failure.  *op* must not be tracked by the collector yet.
 
 
 .. c:function:: void PyObject_GC_Track(PyObject *op)
@@ -110,7 +110,7 @@
    Traversal function for a container object.  Implementations must call the
    *visit* function for each object directly contained by *self*, with the
    parameters to *visit* being the contained object and the *arg* value passed
-   to the handler.  The *visit* function must not be called with a *NULL*
+   to the handler.  The *visit* function must not be called with a ``NULL``
    object argument.  If *visit* returns a non-zero value that value should be
    returned immediately.
 
@@ -121,7 +121,7 @@
 
 .. c:function:: void Py_VISIT(PyObject *o)
 
-   If *o* is not *NULL*, call the *visit* callback, with arguments *o*
+   If *o* is not ``NULL``, call the *visit* callback, with arguments *o*
    and *arg*.  If *visit* returns a non-zero value, then return it.
    Using this macro, :c:member:`~PyTypeObject.tp_traverse` handlers
    look like::
@@ -134,7 +134,7 @@
           return 0;
       }
 
-The :c:member:`~PyTypeObject.tp_clear` handler must be of the :c:type:`inquiry` type, or *NULL*
+The :c:member:`~PyTypeObject.tp_clear` handler must be of the :c:type:`inquiry` type, or ``NULL``
 if the object is immutable.