Edit documentation of 'system-tick(). The previous description has been construed by some to assert that it is unsafe to be called from anywhere except a 'naked' ISR.

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2349 1084037a-409f-4a99-8160-1a4b3577d4c1