Add ignoreCase doc for assertContains

After looking up online different ugly solutions (use of regex, normalizing strings, ...) I realized that this bit existed in the [API][1] but not in the [current documentation][2].

I only added the piece of doc for version 4.2 in order to get review, but if the review says it's good to go, I can add this to every version in the docs.

Also, if someone provides me the translated bits in other languages, I can add them.

My editor removed trailing whitespaces, sorry for the noise.

[1]: http://apigen.juzna.cz/doc/sebastianbergmann/phpunit/function-assertContains.html
[2]: http://phpunit.de/manual/4.2/en/appendixes.assertions.html#appendixes.assertions.assertContains