doc/next: document go mod tidy -diff flag

Add a release note for the flag that CL 585401 added.

For #27005.
For #65614.

Change-Id: Ib26eb1b85c511d3cb41a29a8d9354577dd9a0e14
Reviewed-on: https://go-review.googlesource.com/c/go/+/587695
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

Author: dmitshur

doc/next/3-tools.md

@@ -8,10 +8,16 @@ Distributions that install the `go` command to a location other than
 `$GOROOT/bin/go` should install a symlink instead of relocating
 or copying the `go` binary.
 
-The new go env `-changed` flag causes the command to print only
+<!-- go.dev/issue/34208, CL 563137, CL 586095 -->
+The new `go` `env` `-changed` flag causes the command to print only
 those settings whose effective value differs from the default value
 that would be obtained in an empty environment with no prior uses of the `-w` flag.
 
+<!-- go.dev/issue/27005, CL 585401 -->
+The new `go` `mod` `tidy` `-diff` flag causes the command not to modify
+the files but instead print the necessary changes as a unified diff.
+It exits with a non-zero code if updates are needed.
+
 ### Vet {#vet}
 
 The `go vet` subcommand now includes the