[WCM] [SecurityBundle] Document multiple IP matching in rules

| Q             | A
| ------------- | ---
| Doc fix?      | No
| New docs?     | Yes, for pull request symfony/symfony#7251
| Applies to    | 2.3 (I think, this pull request and the code pull request are against master)
| Fixed tickets | See above

This is the documentation for the [pull request for multiple IP address matching in the request matcher against symfony](https://github.com/symfony/symfony/pull/7251).

I'm not sure exactly what Doc fix vs. new docs means but I basically just updated the example in the book to have 1 example with multiple IPs and added a ``.. versionadded`` notice.  I wasn't sure if the changelog should be updated as well -- I couldn't tell if it was the changelog for the symfony code or for the documentation.