Reworked (yet again) the description of system_tick(). I don't know how to describe it any more clearly than it is now.
Also removed my time zone from the figure for the 'end of time'.

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2355 1084037a-409f-4a99-8160-1a4b3577d4c1