Improve documentation of 'run-at-time'

Improve documentation of 'run-at-time'

* lisp/emacs-lisp/timer.el (run-at-time): Improve the doc string.
In particular, don't refer to 'diary-entry-time', because it is
unavailable until diary-lib is loaded.  Also, refer to
'timer-duration-words', not 'timer-duration', as the latter's doc
string says nothing about the accepted strings.