cmd/go: document the build flags properly for vet

The "build flags" mentioned in the documentation are only those
that apply to analyzing packages and executing the tool.

Fixes #21711.

Change-Id: Ie7b2a354f1e30c928b40888c51fc68e599a5444a
Reviewed-on: https://go-review.googlesource.com/60830Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -808,7 +808,9 @@
 // The -n flag prints commands that would be executed.
 // The -x flag prints commands as they are executed.
 //
-// For more about build flags, see 'go help build'.
+// The build flags supported by go vet are those that control package resolution
+// and execution, such as -n, -x, -v, -tags, and -toolexec.
+// For more about these flags, see 'go help build'.
 //
 // See also: go fmt, go fix.
 //

src/cmd/go/internal/vet/vet.go

@@ -28,7 +28,9 @@ For more about specifying packages, see 'go help packages'.
 The -n flag prints commands that would be executed.
 The -x flag prints commands as they are executed.
 
-For more about build flags, see 'go help build'.
+The build flags supported by go vet are those that control package resolution
+and execution, such as -n, -x, -v, -tags, and -toolexec.
+For more about these flags, see 'go help build'.
 
 See also: go fmt, go fix.
 	`,