os: document that OpenRoot follows symlinks

OpenRoot is expected to be called with a trusted path,
and does not attempt to defend against symlinks in that path.

Fixes #71806

Change-Id: Ib8b2e123e323d22d5c23ed9f711d21995139a7ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/650355
Auto-Submit: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: neild

src/os/root.go

@@ -76,6 +76,7 @@ const (
 )
 
 // OpenRoot opens the named directory.
+// It follows symbolic links in the directory name.
 // If there is an error, it will be of type *PathError.
 func OpenRoot(name string) (*Root, error) {
 	testlog.Open(name)