content/static/doc: document 'go mod graph'

For golang/go#33637

Change-Id: Idb7006881b75394f2ffbfb6861fc8e54f19888ed
Reviewed-on: https://go-review.googlesource.com/c/website/+/284588
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

content/static/doc/mod.md

@@ -1415,6 +1415,41 @@ Tools may also use the package
 [`golang.org/x/mod/modfile`](https://pkg.go.dev/golang.org/x/mod/modfile?tab=doc)
 to parse, edit, and format `go.mod` files.
 
+### `go mod graph` {#go-mod-graph}
+
+Usage:
+
+```
+go mod graph
+```
+
+The `go mod graph` command prints the [module requirement
+graph](#glos-module-graph) (with replacements applied) in text form. For
+example:
+
+```
+example.com/main example.com/a@v1.1.0
+example.com/main example.com/b@v1.2.0
+example.com/a@v1.1.0 example.com/b@v1.1.1
+example.com/a@v1.1.0 example.com/c@v1.3.0
+example.com/b@v1.1.0 example.com/c@v1.1.0
+example.com/b@v1.2.0 example.com/c@v1.2.0
+```
+
+Each vertex in the module graph represents a specific version of a module.
+Each edge in the graph represents a requirement on a minimum version of a
+dependency.
+
+`go mod graph` prints the edges of the graph, one per line. Each line has two
+space-separated fields: a module version and one of its dependencies. Each
+module version is identified as a string of the form `path@version`. The main
+module has no `@version` suffix, since it has no version.
+
+See [Minimal version selection (MVS)](#minimal-version-selection) for more
+information on how versions are chosen. See also [`go list -m`](#go-list-m) for
+printing selected versions and [`go mod why`](#go-mod-why) for understanding
+why a module is needed.
+
 ### `go mod init` {#go-mod-init}
 
 Usage: