Property access tweaks

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.5+
| Fixed tickets | #3729

Hi guys!

This follows after #3729 - it includes several small suggestions made by @WouterJ and @xabbuh.

Additionally, this removes the 3rd argument to `isWritable`, which was removed in the orignal PR (symfony/symfony#10570) and didn't make it into the final feature.

Thanks!