8306431: File.listRoots method description should be re-examined

Reviewed-by: bpb, alanb

src/java.base/share/classes/java/nio/file/FileSystem.java

@@ -178,7 +178,8 @@ public abstract class FileSystem
      * hierarchies may change during the lifetime of the Java virtual machine.
      * For example, in some implementations, the insertion of removable media
      * may result in the creation of a new file hierarchy with its own
-     * top-level directory.
+     * top-level directory. There is no guarantee that a root directory
+     * can be accessed.
      *
      * <p> When a security manager is installed, it is invoked to check access
      * to the each root directory. If denied, the root directory is not returned