Refactored OptionsResolver docs to have a better example

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.1+
| Fixed tickets | https://github.com/symfony/symfony-docs/pull/2353#issuecomment-16654936

As requested by @bschussek, I changed the example to a `Mailer` class. There are only 2 things I don't like:

 1. The example under *"Default Values that depend on another Option"* shows a `port` setting depending on the `host` setting. I think it's a really weird example, but I can't think of a better one.

 2. I can't find a good example for the `replaceDefaults` method.