added information on AuthenticationFailureHandlerInterface in api keys docs
fixed line wrapping
fixed authentication failed header
recommendations by xabbuh
fixed typo
feature #3565 added information on AuthenticationFailureHandlerInterface (samsamm777)

This PR was submitted for the master branch but it was merged into the 2.4 branch instead (closes #3565).

Discussion
----------

added information on AuthenticationFailureHandlerInterface

The Api Keys documentation made no mention of the AuthenticationFailureHandlerInterface which is required to correctly display Authentication Failure responses. Without it, authentication failures will result in a 500 response. I've made mention to the interface and given an example implementation.

http://symfony.com/doc/current/cookbook/security/api_key_authentication.html#cookbook-security-api-key-config

```
Doc fix? yes
New docs? no
Applies to: 2.4
Fixed tickets: none found
```

Commits
-------

066bccb fixed typo
db8e01a recommendations by xabbuh
3366dfc fixed authentication failed header
21e3536 fixed line wrapping
e4f5c6e added information on AuthenticationFailureHandlerInterface in api keys docs