doc: add release notes for runtime-internal contention

For #57071.
For #61422.

Change-Id: I5d546d8828be897cb087e85a1251213c582b3894
Reviewed-on: https://go-review.googlesource.com/c/go/+/547059
Reviewed-by: Rhys Hiltner <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: prattmic

doc/go1.22.html

@@ -638,6 +638,13 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
   <dd>
+    <p><!-- https://go.dev/issue/57071 -->
+      <code>/sync/mutex/wait/total:seconds</code> now includes contention on
+      runtime-internal locks in addition to
+      <a href="/pkg/sync#Mutex"><code>sync.Mutex</code></a> and
+      <a href="/pkg/sync#RWMutex"><code>sync.RWMutex</code></a>.
+    </p>
+
     <p><!-- https://go.dev/issue/63340 -->
       TODO: <a href="https://go.dev/issue/63340">https://go.dev/issue/63340</a>: provide histogram of all STW events
     </p>
@@ -654,6 +661,14 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       now record 1 second of delay instead of 10 milliseconds of delay.
     </p>
 
+    <p><!-- https://go.dev/issue/57071 -->
+      Mutex profiles also now include contention on runtime-internal locks in addition to
+      <a href="/pkg/sync#Mutex"><code>sync.Mutex</code></a> and
+      <a href="/pkg/sync#RWMutex"><code>sync.RWMutex</code></a>.
+      Contention on runtime-internal locks is always reported at <code>runtime._LostContendedRuntimeLock</code>.
+      A future release will add complete stack traces in these cases.
+    </p>
+
     <p><!-- https://go.dev/issue/50891 -->
       CPU profiles on Darwin platforms now contain the process's memory map, enabling the disassembly
       view in the pprof tool.