relnotes/13.0: add note about jail(8) cpuset behavior

Attaching processes will now largely retain their cpu masks, which is a
fairly large and notable change from previous releases.

Reviewed by:	me_igalic.co
Approved by:	gjb (implicit)
Differential Revision:	https://reviews.freebsd.org/D29081

website/content/en/releases/13.0R/relnotes.adoc

@@ -157,6 +157,10 @@ This section covers changes to kernel configurations, system tuning, and system
 [[kernel-general]]
 === General Kernel Changes
 
+Processes that attach to a {{< manpage "jail" "8" >}} will now completely rebase their {{< manpage "cpuset" "1" >}} onto the jail's cpuset.
+Notably, if a process had been assigned a numbered cpuset then it will be assigned a new numbered set that is the combination of CPUs
+allowed to the attaching process and the jail.
+Processes belonging to the superuser will implicitly widen their CPU mask as needed if they share no CPUs in common with the jail.
 
 [[drivers]]
 == Devices and Drivers