bug#25798: 24.5; Doc string of `run-at-time'

[Drew Adams]

The doc string does not say that TIME can be a number, which it
apparently can.  The doc string currently says that TIME can
("should", which is a waffle word) only be a string, nil, or t.

The doc should say that it can be a number, and it should say
just how a number value is interpreted.

This code in `run-at-time' shows that TIME can be a number:

(if (numberp time)
    (setq time (timer-relative-time (current-time) time)))

And other code in Emacs makes use of a numeric TIME value.
E.g., this code in window.el:

(run-at-time
  (abs mouse-autoselect-window) nil 'mouse-autoselect-window-select)

The doc in the manual correctly specifies that TIME can be a
number:

If TIME is a number (integer or floating point), that specifies
a relative time measured in seconds.  

In GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --host=i686-pc-mingw32'