[Components][OptionsResolver] Fix&improve replaceDefaults() description

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | 2.2+
| Fixed tickets | -

The description of the `replaceDefaults()` method was not correct. This PR replaces it with a new section about how to overwrite or replace default values correctly. See also:

* http://api.symfony.com/2.2/Symfony/Component/OptionsResolver/OptionsResolver.html#method_setDefaults
* http://api.symfony.com/2.2/Symfony/Component/OptionsResolver/OptionsResolver.html#method_replaceDefaults

Also changed *Closure* to *closure* to be consistent (mixed usage before, *closure* follows Wikipedia)