doc/next: document GORISCV64 environment variable

For #65614.
Updates #61476.

Change-Id: Id677aa6d2a59366ab75a26f08a383d2d253f270e
Reviewed-on: https://go-review.googlesource.com/c/go/+/591895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

doc/next/7-ports.md

@@ -24,6 +24,13 @@ Go 1.23 introduces a new `GOARM64` environment variable, which specifies the min
 
 The `GOARM64` environment variable defaults to `v8.0`.
 
+### RISC-V {#riscv}
+
+<!-- go.dev/issue/61476, CL 541135 -->
+Go 1.23 introduces a new `GORISCV64` environment variable, which selects the [RISC-V user-mode application profile](https://github.com/riscv/riscv-profiles/blob/main/src/profiles.adoc) for which to compile. Allowed values are `rva20u64` and `rva22u64`.
+
+The `GORISCV64` environment variable defaults to `rva20u64`.
+
 ### 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/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.
-
 <!-- These items need to be reviewed, and mentioned in the Go 1.23 release notes if applicable.
 
 None right now; more may come up later on.