NEWS: document kernel.pid_max change

NEWS

@@ -1,5 +1,26 @@
 systemd System and Service Manager
 
+CHANGES WITH 243 in spe:
+
+        * The "kernel.pid_max" sysctl is now bumped to 4194304 by default,
+          i.e. the full 22bit range the kernel allows, up from the old 16bit
+          range. This should improve security and robustness a bit, as PID
+          collisions are made less likely (though certainly still
+          possible). There are rumours this might create compatibility
+          problems, though at this moment no practical ones are known to
+          us. Downstream distributions are hence advised to undo this change in
+          their builds if they are concerned about maximum compatibility, but
+          for everybody else we recommend leaving the value bumped. Besides
+          improving security and robustness this should also simplify things as
+          the maximum number of allowed concurrent tasks was previously bounded
+          by both "kernel.pid_max" and "kernel.threads-max" and now only a
+          single knob is left ("kernel.threads-max"). There have been concerns
+          that usability is affected by this change because larger PID numbers
+          are harder to type, but we believe the change from 5 digit PIDs to 7
+          digit PIDs is not too hampering for usability.
+
+          …
+
 CHANGES WITH 242:
 
         * In .link files, MACAddressPolicy=persistent (the default) is changed