doc/go1.21: document GOARM default when cross-compiling

This was changed in CL 470695 but was not mentioned in the release notes.

For golang/go#24904
For golang/go#58884

Change-Id: Ib952ce43c65696b3a6091a378587cab1299f30e3
Reviewed-on: https://go-review.googlesource.com/c/website/+/639455
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

_content/doc/go1.21.md

@@ -1118,9 +1118,21 @@ As [announced](go1.20#windows) in the Go 1.20 release notes,
 Go 1.21 requires at least Windows 10 or Windows Server 2016;
 support for previous versions has been discontinued.
 
+### ARM
+
 <!-- CL 470695 -->
 
-<!-- cmd/dist: default to GOARM=7 on all non-arm systems -->
+When building the Go distribution with `GOARCH=arm` when not running
+on an ARM system (that is, when building a cross-compiler to ARM), the
+default value for the `GOARM` environment variable is now always set
+to `7`.
+Previously the default depended on characteristics of the build
+system.
+
+When not building a cross-compiler, the default value is determined
+by examining the build system.
+That was true before and remains true in Go 1.21.
+What has changed is the behavior when building a cross-compiler.
 
 ### WebAssembly {#wasm}
 

_content/doc/go1.22.md

@@ -868,7 +868,7 @@ the Go toolchain already generates PIE by default.
 <!-- go.dev/issue/64207 -->
 Go 1.22 is the last release that will run on macOS 10.15 Catalina. Go 1.23 will require macOS 11 Big Sur or later.
 
-### Arm {#arm}
+### ARM {#arm}
 
 <!-- CL 514907 -->
 The `GOARM` environment variable now allows you to select whether to use software or hardware floating point.