doc/next: add release note for new toolchain line behavior

The go command will now no longer update the toolchain line implicitly
to the local toolchain version when updating the go line. Document that
in a release note.

For #65847

Change-Id: I4e970d881a43c22292fe9fa65a9835d0214ef7bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/657178
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: matloob

doc/next/3-tools.md

@@ -10,5 +10,11 @@ These new error reports may be disabled by setting
 `ASAN_OPTIONS=detect_leaks=0` in the environment when running the
 program.
 
+<!-- go.dev/issue/65847 -->
+
+When the go command updates the `go` line in a `go.mod` or `go.work` file,
+it [no longer](/ref/mod#go-mod-file-toolchain) adds a toolchain line
+specifying the command's current version.
+
 ### Cgo {#cgo}