Mention the systemd issue with old-style callbacks (#74)

* Mention the systemd issue with old-style callbacks

* Copy the whole explanation

Author: arigo

doc/source/using.rst

@@ -891,6 +891,10 @@ ffi.callback() and the result is the same.
     - `On Mac OS X,`__ you need to give your application the entitlement
       ``com.apple.security.cs.allow-unsigned-executable-memory``.
 
+    - On Linux, ``systemd`` installs syscall filtering rules on services
+      it supervises.   The `MemoryDenyWriteExecute=` setting in
+      `systemd.exec(5)` defaults to on, and can quietly block this.
+
     Note also that a cffi fix for this issue was attempted---see
     the ``ffi_closure_alloc`` branch---but was not merged because it
     creates potential `memory corruption`__ with ``fork()``.