relnote: add ARM64 spinlock item

Reported-by: Jonathan Katz

Discussion: per private email

Author: bmomjian

doc/src/sgml/release-15.sgml

@@ -838,6 +838,17 @@ Author: David Rowley <drowley@postgresql.org>
 <para>
 Improve the performance of window functions that use row_number(), rank(), and count() (David Rowley)
 </para>
+</listitem>
+
+<!--
+Author: Tom Lane <tgl@sss.pgh.pa.us>
+2022-04-06 [a82a5eee3] Use ISB as a spin-delay instruction on ARM64.
+-->
+
+<listitem>
+<para>
+Improve the performance of spinlocks on high-core-count ARM64 systems (Geoffrey Blake)
+</para>
 </listitem>
 
      </itemizedlist>