doc: add release notes for PGO improvements

For #61577.
For #61422.

Change-Id: I575bf657fb36bd7103c73620bb2371d0f490af20
Reviewed-on: https://go-review.googlesource.com/c/go/+/546935
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

doc/go1.22.html

@@ -124,7 +124,14 @@ <h2 id="runtime">Runtime</h2>
 <h2 id="compiler">Compiler</h2>
 
 <p>
-  TODO: complete this section, or delete if not needed
+  TODO: complete this section.
+</p>
+
+<p><!-- https://go.dev/issue/61577 -->
+  <a href="https://go.dev/doc/pgo">Profile-guided Optimization (PGO)</a> builds
+  can now devirtualize a higher proportion of calls than previously possible.
+  Most programs from a representative set of Go programs now see between 2 and
+  14% improvement from enabling PGO.
 </p>
 
 <h2 id="linker">Linker</h2>