cmd/go: document 'go install cmd@version' ignores vendor directories

For #48332

Change-Id: I708eb3e8f3f386f03210b7117d9ab8b0be2125bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/349591
Trust: Jay Conrod <[email protected]>
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: Jay Conrod

src/cmd/go/alldocs.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