doc/next: document GOARM64 environment variable

For #65614.
Updates #60905.

Change-Id: I2dd9df3c7066357cf06268d918bad3c255b38aed
Reviewed-on: https://go-review.googlesource.com/c/go/+/591875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>

doc/next/7-ports.md

@@ -17,6 +17,13 @@ Go 1.23 is the last release that requires Linux kernel version 2.6.32 or later.
 <!-- go.dev/issue/55999, CL 518629, CL 518630 -->
 Go 1.23 adds experimental support for OpenBSD on 64-bit RISC-V (`GOOS=openbsd`, `GOARCH=riscv64`).
 
+### ARM64 {#arm64}
+
+<!-- go.dev/issue/60905, CL 559555 -->
+Go 1.23 introduces a new `GOARM64` environment variable, which specifies the minimum target version of the ARM64 architecture at compile time. Allowed values are `v8.{0-9}` and `v9.{0-5}`. This may be followed by an option specifying extensions implemented by target hardware. Valid options are `,lse` and `,crypto`.
+
+The `GOARM64` environment variable defaults to `v8.0`.
+
 ### Wasm {#wasm}
 
 <!-- go.dev/issue/63718 -->

doc/next/9-todo.md

@@ -1,8 +1,5 @@
 <!-- These items need to be completed and moved to an appropriate location in the release notes. -->
 
-<!-- go.dev/issue/60905, CL 559555 -->
-TODO: The new `GOARM64` environment variable needs to be documented. This note should be moved to an appropriate location in the release notes.
-
 <!-- go.dev/issue/61476, CL 541135 -->
 TODO: The new `GORISCV64` environment variable needs to be documented. This note should be moved to an appropriate location in the release notes.