[3.8] bpo-38600: Change the mark up of NULL in the C API documentation. (GH-16950) (GH-16999)

Replace all *NULL* with ``NULL``.
(cherry picked from commit 25fc088607c855060ed142296dc1bd0125fad1af)
diff --git a/Doc/c-api/reflection.rst b/Doc/c-api/reflection.rst
index 080ea32..1d86de6 100644
--- a/Doc/c-api/reflection.rst
+++ b/Doc/c-api/reflection.rst
@@ -14,18 +14,18 @@
 .. c:function:: PyObject* PyEval_GetLocals()
 
    Return a dictionary of the local variables in the current execution frame,
-   or *NULL* if no frame is currently executing.
+   or ``NULL`` if no frame is currently executing.
 
 
 .. c:function:: PyObject* PyEval_GetGlobals()
 
    Return a dictionary of the global variables in the current execution frame,
-   or *NULL* if no frame is currently executing.
+   or ``NULL`` if no frame is currently executing.
 
 
 .. c:function:: PyFrameObject* PyEval_GetFrame()
 
-   Return the current thread state's frame, which is *NULL* if no frame is
+   Return the current thread state's frame, which is ``NULL`` if no frame is
    currently executing.