bug#27494: 24.5; doc of `add-to-history'

[Eli Zaretskii]

> Date: Sun, 25 Jun 2017 17:08:18 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> Both doc string and Elisp manual:
> 
> The doc should say that when MAXELT is exceeded the oldest history
> element is discarded.  Yes, one might surmise that from the meaning of
> "history" ,and from assuming that the new element is always added,
> even if the history is full (up to MAXELT).  Still, it is better to be
> explicit.

The ELisp manual already said that the values of maximum length have
the same meaning as those of 'history-length', so I just added the
same reference to the doc string.

Thanks.