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/buffer.rst b/Doc/c-api/buffer.rst
index e660716..f37b0db 100644
--- a/Doc/c-api/buffer.rst
+++ b/Doc/c-api/buffer.rst
@@ -102,13 +102,13 @@
    .. c:member:: void \*obj
 
       A new reference to the exporting object. The reference is owned by
-      the consumer and automatically decremented and set to *NULL* by
+      the consumer and automatically decremented and set to ``NULL`` by
       :c:func:`PyBuffer_Release`. The field is the equivalent of the return
       value of any standard C-API function.
 
       As a special case, for *temporary* buffers that are wrapped by
       :c:func:`PyMemoryView_FromBuffer` or :c:func:`PyBuffer_FillInfo`
-      this field is *NULL*. In general, exporting objects MUST NOT
+      this field is ``NULL``. In general, exporting objects MUST NOT
       use this scheme.
 
    .. c:member:: Py_ssize_t len
@@ -134,21 +134,21 @@
 
       Important exception: If a consumer requests a buffer without the
       :c:macro:`PyBUF_FORMAT` flag, :c:member:`~Py_buffer.format` will
-      be set to  *NULL*,  but :c:member:`~Py_buffer.itemsize` still has
+      be set to  ``NULL``,  but :c:member:`~Py_buffer.itemsize` still has
       the value for the original format.
 
       If :c:member:`~Py_buffer.shape` is present, the equality
       ``product(shape) * itemsize == len`` still holds and the consumer
       can use :c:member:`~Py_buffer.itemsize` to navigate the buffer.
 
-      If :c:member:`~Py_buffer.shape` is *NULL* as a result of a :c:macro:`PyBUF_SIMPLE`
+      If :c:member:`~Py_buffer.shape` is ``NULL`` as a result of a :c:macro:`PyBUF_SIMPLE`
       or a :c:macro:`PyBUF_WRITABLE` request, the consumer must disregard
       :c:member:`~Py_buffer.itemsize` and assume ``itemsize == 1``.
 
    .. c:member:: const char \*format
 
       A *NUL* terminated string in :mod:`struct` module style syntax describing
-      the contents of a single item. If this is *NULL*, ``"B"`` (unsigned bytes)
+      the contents of a single item. If this is ``NULL``, ``"B"`` (unsigned bytes)
       is assumed.
 
       This field is controlled by the :c:macro:`PyBUF_FORMAT` flag.
@@ -158,7 +158,7 @@
       The number of dimensions the memory represents as an n-dimensional array.
       If it is ``0``, :c:member:`~Py_buffer.buf` points to a single item representing
       a scalar. In this case, :c:member:`~Py_buffer.shape`, :c:member:`~Py_buffer.strides`
-      and :c:member:`~Py_buffer.suboffsets` MUST be *NULL*.
+      and :c:member:`~Py_buffer.suboffsets` MUST be ``NULL``.
 
       The macro :c:macro:`PyBUF_MAX_NDIM` limits the maximum number of dimensions
       to 64. Exporters MUST respect this limit, consumers of multi-dimensional
@@ -248,7 +248,7 @@
    .. c:macro:: PyBUF_FORMAT
 
       Controls the :c:member:`~Py_buffer.format` field. If set, this field MUST
-      be filled in correctly. Otherwise, this field MUST be *NULL*.
+      be filled in correctly. Otherwise, this field MUST be ``NULL``.
 
 
 :c:macro:`PyBUF_WRITABLE` can be \|'d to any of the flags in the next section.
@@ -349,9 +349,9 @@
 
 If ``ndim == 0``, the memory location pointed to by :c:member:`~Py_buffer.buf` is
 interpreted as a scalar of size :c:member:`~Py_buffer.itemsize`. In that case,
-both :c:member:`~Py_buffer.shape` and :c:member:`~Py_buffer.strides` are *NULL*.
+both :c:member:`~Py_buffer.shape` and :c:member:`~Py_buffer.strides` are ``NULL``.
 
-If :c:member:`~Py_buffer.strides` is *NULL*, the array is interpreted as
+If :c:member:`~Py_buffer.strides` is ``NULL``, the array is interpreted as
 a standard n-dimensional C-array. Otherwise, the consumer must access an
 n-dimensional array as follows:
 
@@ -438,7 +438,7 @@
 
    Send a request to *exporter* to fill in *view* as specified by  *flags*.
    If the exporter cannot provide a buffer of the exact type, it MUST raise
-   :c:data:`PyExc_BufferError`, set :c:member:`view->obj` to *NULL* and
+   :c:data:`PyExc_BufferError`, set :c:member:`view->obj` to ``NULL`` and
    return ``-1``.
 
    On success, fill in *view*, set :c:member:`view->obj` to a new reference
@@ -518,7 +518,7 @@
 
    On success, set :c:member:`view->obj` to a new reference to *exporter* and
    return 0. Otherwise, raise :c:data:`PyExc_BufferError`, set
-   :c:member:`view->obj` to *NULL* and return ``-1``;
+   :c:member:`view->obj` to ``NULL`` and return ``-1``;
 
    If this function is used as part of a :ref:`getbufferproc <buffer-structs>`,
    *exporter* MUST be set to the exporting object and *flags* must be passed