doc: document WER and SEH changes in Go 1.21

While here, I've removed the CL 472195 TODO, which I marked as
RELNOTE=yes by mistake.

For #57441
For #57302

Change-Id: I7563140bf307f8d732f0154d02b8fa0735527323
Reviewed-on: https://go-review.googlesource.com/c/go/+/499515
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Run-TryBot: Quim Muntal <[email protected]>

Author: qmuntal

doc/go1.21.html

@@ -231,6 +231,12 @@ <h2 id="assembler">Assembler</h2>
 
 <h2 id="linker">Linker</h2>
 
+<p><!-- https://go.dev/issue/57302, CL 461749 -->
+  On Windows AMD64, the linker (with help from the compiler) now emits
+  SEH unwinding data by default, which improves the integration
+  of Go applications with Windows debuggers and other tools.
+</p>
+
 <!-- CL 457455 -->
 <p>
   <!-- cmd/link: generate .xdata PE section -->
@@ -745,8 +751,11 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       TODO: <a href="https://go.dev/issue/38651">https://go.dev/issue/38651</a>: add &#39;created by goroutine number&#39; to stack traces
     </p>
 
-    <p><!-- https://go.dev/issue/57441 -->
-      TODO: <a href="https://go.dev/issue/57441">https://go.dev/issue/57441</a>: use WER for GOTRACEBACK=wer on Windows
+    <p><!-- https://go.dev/issue/57441, CL 474915 -->
+      Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
+      <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
+      before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
+      On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
     </p>
 
     <p><!-- CL 447778 -->
@@ -758,14 +767,6 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
     </p>
 
-    <p><!-- CL 472195 -->
-      TODO: <a href="https://go.dev/cl/472195">https://go.dev/cl/472195</a>: runtime: remove NOFRAME from asmcgocall, systemstack and mcall
-    </p>
-
-    <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