docs: Permissions.readonly() also ignores root user special permissions

The root user can write to files without any (write) access bits set. But this is not taken into account by `std::fs::Permissions.readonly()`.

Author: intgr

library/std/src/fs.rs

@@ -1564,8 +1564,8 @@ impl Permissions {
     ///
     /// # Note
     ///
-    /// This function does not take Access Control Lists (ACLs) or Unix group
-    /// membership into account.
+    /// This function does not take Access Control Lists (ACLs), Unix group
+    /// membership or Unix root user special permissions into account.
     ///
     /// # Windows
     ///