doc: write various runtime-related release notes

This includes release notes for several small runtime changes, including
runtime/trace and runtime package changes.

For #58645.

Change-Id: I3e9c804da1bb6b385088e16a20d9576c11098021
Reviewed-on: https://go-review.googlesource.com/c/go/+/497095
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ansiwen <ansiwen@gmail.com>
Reviewed-by: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

doc/go1.21.html

@@ -51,6 +51,8 @@ Do not send CLs removing the interior tags from such phrases.
   <!-- replace CallImport with go:wasmimport directive -->
 </p>
 
+<h3 id="wasip1">WebAssembly</h3>
+
 <!-- https://go.dev/issue/59149 -->
 <p>
   <!-- allow the use of go:wasmimport globally -->
@@ -61,6 +63,13 @@ Do not send CLs removing the interior tags from such phrases.
   <!-- cmd/compile: remove go:wasmimport restriction -->
 </p>
 
+<!-- https://go.dev/issue/56100 -->
+<p>
+  The Go scheduler now interacts much more efficiently with the
+  JavaScript event loop, especially in applications that block
+  frequently on asynchronous events.
+</p>
+
 
 <h3 id="wasip1">WebAssembly System Interface</h3>
 
@@ -121,6 +130,27 @@ Do not send CLs removing the interior tags from such phrases.
   valuable for debugging stack overflows.
 </p>
 
+<p><!-- https://go.dev/issue/59960 -->
+  On Linux platforms that support transparent huge pages, the Go runtime
+  now manages which parts of the heap may be backed by huge pages more
+  explicitly. This leads to better utilization of memory: small heaps
+  should see less memory used (up to 50% in pathological cases) while
+  large heaps should see fewer broken huge pages for dense parts of the
+  heap, improving CPU usage and latency by up to 1%.
+</p>
+
+<p><!-- https://go.dev/issue/57069, https://go.dev/issue/56966 -->
+  As a result of runtime-internal garbage collection tuning,
+  applications may see up to a 40% reduction in application tail latency
+  and a small decrease in memory use. Some applications may also observe
+  a small loss in throughput.
+
+  The memory use decrease should be proportional to the loss in
+  throughput, such that the previous release's throughput/memory
+  tradeoff may be recovered (with little change to latency) by
+  increasing <code>GOGC</code> and/or <code>GOMEMLIMIT</code> slightly.
+</p>
+
 <h2 id="compiler">Compiler</h2>
 
 <!-- CL 490819 -->
@@ -638,9 +668,35 @@ Do not send CLs removing the interior tags from such phrases.
     <p><!-- CL 474915 -->
       TODO: <a href="https://go.dev/cl/474915">https://go.dev/cl/474915</a>: runtime: support GOTRACEBACK=wer on Windows
     </p>
+
+    <p><!-- https://go.dev/issue/46787 -->
+      A new type <code>Pinner</code> has been added to the runtime
+      package. <code>Pinner</code>s may be used to "pin" Go memory
+      such that it may be used more freely by non-Go code. For instance,
+      passing Go values that reference pinned Go memory to C code is
+      now allowed. Previously, passing any such nested reference was
+      disallowed by the
+      <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>
+
+      See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
+    </p>
   </dd>
 </dl><!-- runtime -->
 
+<dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
+  <dd>
+    <p><!-- https://go.dev/issue/16638 -->
+      Collecting traces on amd64 and arm64 now incurs a substantially
+      smaller CPU cost: up to a 10x improvement over the previous release.
+    </p>
+
+    <p><!-- CL 494495 -->
+      Traces now contain explicit stop-the-world events for every reason
+      the Go runtime might stop-the-world, not just garbage collection.
+    </p>
+  </dd>
+</dl><!-- runtime/trace -->
+
 <dl id="slices"><dt><a href="/pkg/slices/">slices</a></dt>
   <dd>
     <p><!-- https://go.dev/issue/54768 -->