_content/ref/mod: add -x and -diff flags to go mod tidy

Existing documentation for go mod tidy is missing -x and -diff flags.
Adds these flags to the documentation.

Fixes golang/go#69936

Change-Id: I00764066684461d38684b5711a17483819b50904
GitHub-Last-Rev: 8266f04
GitHub-Pull-Request: #304
Reviewed-on: https://go-review.googlesource.com/c/website/+/639275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

Author: willboland

_content/ref/mod.md

@@ -2272,7 +2272,7 @@ requirements and to drop unused requirements.
 Usage:
 
 ```
-go mod tidy [-e] [-v] [-go=version] [-compat=version]
+go mod tidy [-e] [-v] [-x] [-diff] [-go=version] [-compat=version]
 ```
 
 `go mod tidy` ensures that the `go.mod` file matches the source code in the
@@ -2287,6 +2287,12 @@ despite errors encountered while loading packages.
 The `-v` flag causes `go mod tidy` to print information about removed modules
 to standard error.
 
+The `-x` flag causes `go mod tidy` to print the commands `tidy` executes.
+
+The `-diff` flag causes `go mod tidy` not to modify go.mod or go.sum but
+instead print the necessary changes as a unified diff. It exits
+with a non-zero code if the diff is not empty.
+
 `go mod tidy` works by loading all of the packages in the [main
 module](#glos-main-module), all of its tools, and all of the packages they import,
 recursively. This includes packages imported by tests (including tests in other