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.

lisp/emacs-lisp/timer.el

@@ -345,22 +345,23 @@ This function is called, by name, directly by the C code."
 (defun run-at-time (time repeat function &rest args)
   "Perform an action at time TIME.
 Repeat the action every REPEAT seconds, if REPEAT is non-nil.
+REPEAT may be an integer or floating point number.
 TIME should be one of:
-- a string giving an absolute time like \"11:23pm\" (the
-  acceptable formats are those recognized by
-  `diary-entry-time'; note that such times are interpreted
-  as times today, even if in the past);
-- a string giving a relative time like \"2 hours 35 minutes\"
-  (the acceptable formats are those recognized by
-  `timer-duration');
-- nil meaning now;
+- a string giving today's time like \"11:23pm\"
+  (the acceptable formats are HHMM, H:MM, HH:MM, HHam, HHAM,
+  HHpm, HHPM, HH:MMam, HH:MMAM, HH:MMpm, or HH:MMPM;
+  a period '.' can be used instead of a colon ':' to separate
+  the hour and minute parts);
+- a string giving a relative time like \"90\" or \"2 hours 35 minutes\"
+  (the acceptable forms are a number of seconds without units
+  or some combination of values using units in `timer-duration-words');
+- nil, meaning now;
 - a number of seconds from now;
 - a value from `encode-time';
 - or t (with non-nil REPEAT) meaning the next integral
   multiple of REPEAT.
 
-REPEAT may be an integer or floating point number.  The
-action is to call FUNCTION with arguments ARGS.
+The action is to call FUNCTION with arguments ARGS.
 
 This function returns a timer object which you can use in
 `cancel-timer'."