Issue #2905109 by iainp999, gawaksh, msankhala, joachim, borisson_, alexpott:...

Issue #2905109 by iainp999, gawaksh, msankhala, joachim, borisson_, alexpott: DrupalKernel::guessApplicationRoot() should document what the assumptions are and what it's doing

(cherry picked from commit 03a3058d)