cmd/go: document the go statement in a go.mod file

Change-Id: I8db276ec371de56871ce3250f27de1d1dee4b473
Reviewed-on: https://go-review.googlesource.com/c/147279
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/alldocs.go

@@ -1604,17 +1604,20 @@
 // verb followed by arguments. For example:
 //
 // 	module my/thing
+// 	go 1.12
 // 	require other/thing v1.0.2
 // 	require new/thing/v2 v2.3.4
 // 	exclude old/thing v1.2.3
 // 	replace bad/thing v1.4.5 => good/thing v1.4.5
 //
-// The verbs are module, to define the module path; require, to require
-// a particular module at a given version or later; exclude, to exclude
-// a particular module version from use; and replace, to replace a module
-// version with a different module version. Exclude and replace apply only
-// in the main module's go.mod and are ignored in dependencies.
-// See https://research.swtch.com/vgo-mvs for details.
+// The verbs are
+// 	module, to define the module path;
+// 	go, to set the expected language version;
+// 	require, to require a particular module at a given version or later;
+// 	exclude, to exclude a particular module version from use; and
+// 	replace, to replace a module version with a different module version.
+// Exclude and replace apply only in the main module's go.mod and are ignored
+// in dependencies.  See https://research.swtch.com/vgo-mvs for details.
 //
 // The leading verb can be factored out of adjacent lines to create a block,
 // like in Go imports:

src/cmd/go/internal/modload/help.go

@@ -393,17 +393,20 @@ no /* */ comments. Each line holds a single directive, made up of a
 verb followed by arguments. For example:
 
 	module my/thing
+	go 1.12
 	require other/thing v1.0.2
 	require new/thing/v2 v2.3.4
 	exclude old/thing v1.2.3
 	replace bad/thing v1.4.5 => good/thing v1.4.5
 
-The verbs are module, to define the module path; require, to require
-a particular module at a given version or later; exclude, to exclude
-a particular module version from use; and replace, to replace a module
-version with a different module version. Exclude and replace apply only
-in the main module's go.mod and are ignored in dependencies.
-See https://research.swtch.com/vgo-mvs for details.
+The verbs are
+	module, to define the module path;
+	go, to set the expected language version;
+	require, to require a particular module at a given version or later;
+	exclude, to exclude a particular module version from use; and
+	replace, to replace a module version with a different module version.
+Exclude and replace apply only in the main module's go.mod and are ignored
+in dependencies.  See https://research.swtch.com/vgo-mvs for details.
 
 The leading verb can be factored out of adjacent lines to create a block,
 like in Go imports: