Document the "trusted_hosts" framework configuration setting

[ianfp]

The trusted_hosts setting was introduced here:

http://symfony.com/blog/security-releases-symfony-2-0-24-2-1-12-2-2-5-and-2-3-3-released

But is not documented here:

http://symfony.com/doc/current/reference/configuration/framework.html

[xabbuh]

see #4121

[javiereguiluz]

@wouterj @xabbuh @weaverryan this issue can be safely closed because it has been solved.

[xabbuh]

@javiereguiluz Are you sure? #4121 isn't merged yet.

[weaverryan]

It was actually solved in #3876 - thanks Javier!

[xabbuh]

@weaverryan #3876 was taken over by @wouterj in #4121 (which isn't merged). There are no changes in the files that are part of the 2.3 branch or any higher branch yet.