[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/memory.rst b/Doc/c-api/memory.rst
index c5b3af6..ba7bd3b 100644
--- a/Doc/c-api/memory.rst
+++ b/Doc/c-api/memory.rst
@@ -424,7 +424,7 @@
 
    Set the memory block allocator of the specified domain.
 
-   The new allocator must return a distinct non-NULL pointer when requesting
+   The new allocator must return a distinct non-``NULL`` pointer when requesting
    zero bytes.
 
    For the :c:data:`PYMEM_DOMAIN_RAW` domain, the allocator must be