bug#24879: [PATCH] Point out the default value of FRAME in the `set-face-*' functions.

[Eli Zaretskii]

> From: Hong Xu <hong@topbug.net>
> Date: Fri,  4 Nov 2016 13:29:10 -0700
> 
>    The following commands and functions mostly provide compatibility
>  with old versions of Emacs.  They work by calling
> -@code{set-face-attribute}.  Values of @code{t} and @code{nil} for
> -their @var{frame} argument are handled just like
> +@code{set-face-attribute}.  Values of @code{t} and @code{nil}
> +(default) for their @var{frame} argument are handled just like
>  @code{set-face-attribute} and @code{face-attribute}.  The commands
>  read their arguments using the minibuffer, if called interactively.

Do we really need to tell people that an omitted argument defaults to
nil?  And if we do, why in this section?

Thanks.