diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index 425aa831d8..b7e8212795 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -702,14 +702,17 @@
 //
 // - All arguments must refer to packages in the same module at the same version.
 //
+// - Package path arguments must refer to main packages. Pattern arguments
+// will only match main packages.
+//
 // - No module is considered the "main" module. If the module containing
 // packages named on the command line has a go.mod file, it must not contain
 // directives (replace and exclude) that would cause it to be interpreted
 // differently than if it were the main module. The module must not require
 // a higher version of itself.
 //
-// - Package path arguments must refer to main packages. Pattern arguments
-// will only match main packages.
+// - Vendor directories are not used in any module. (Vendor directories are not
+// included in the module zip files downloaded by 'go install'.)
 //
 // If the arguments don't have version suffixes, "go install" may run in
 // module-aware mode or GOPATH mode, depending on the GO111MODULE environment
diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index c51dd398c2..3d7c778a7d 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -496,14 +496,17 @@ allowed, even if they refer to the same version.
 
 - All arguments must refer to packages in the same module at the same version.
 
+- Package path arguments must refer to main packages. Pattern arguments
+will only match main packages.
+
 - No module is considered the "main" module. If the module containing
 packages named on the command line has a go.mod file, it must not contain
 directives (replace and exclude) that would cause it to be interpreted
 differently than if it were the main module. The module must not require
 a higher version of itself.
 
-- Package path arguments must refer to main packages. Pattern arguments
-will only match main packages.
+- Vendor directories are not used in any module. (Vendor directories are not
+included in the module zip files downloaded by 'go install'.)
 
 If the arguments don't have version suffixes, "go install" may run in
 module-aware mode or GOPATH mode, depending on the GO111MODULE environment