Improved the description of what resetting a driver means

This documents that only a few actions are allowed on a driver which has been started or reset until a page is visited. This matches the behavior of browsers, where you cannot perform actions on a page before opening one.
A new test is added to ensure that the basic auth is properly reset when resetting the driver.

This relates to the discussion in https://github.com/Behat/MinkZombieDriver/pull/86