Update docs to highlight reset limitation in Selenium about docs

[aik099]

More details:

When script that was using Mink opens pages from different domains and sets cookies on both of them, then calling Session::reset() would only remove cookies from domain belonging to currently opened page.

In general some drivers doesn't allow setting/removing cookies from other than current domain. To get cross-driver consistent behavior the drivers that can manage cookies on different domains, like Zombie, were restricted to current domain only as well.

[stof]

@aik099 note that ZombieDriver resets all cookies properly on reset (the goal for reset is to do the best resetting possible, not to implement the common denominator). So this limitation is only for Selenium, Selenium2 and Sahi.

@j4m3s if you want to contribute, this should probably be documented in https://github.com/minkphp/docs/blob/master/guides/session.rst#resetting-the-session

[j4m3s]

@stof am I right in thinking that scenarios are "soft isolated" by default (i.e. session is reset so cookies are cleared etc)?

My experience with phantomjs suggests not (see also this SO post from someone a while back: http://stackoverflow.com/questions/17279478/when-running-a-behat-login-test-through-phantomjs-it-only-passes-on-the-first-ru) - but I know selenium and Goutte do. I'm trying to find my feet before doing any documentation updating!

[stof]

@j4m3s yes, it is the case